Not a Chapter Skip this

If you want to read this for some reason it is just an introduction to chemistry with a few missing ages

Introduction

Socrates (469 B.C. - 399 B.C.), Plato (427

B.C. - 347 B.C.), and Aristotle (384 B.C. - 322 B.C.)

are among the most famous of the Greek

philosophers. Plato was a student of Socrates, and

Aristotle was a student of Plato. These three were

probably the greatest thinkers of their time. Aristotle's

views on physical science profoundly shaped

medieval scholarship, and his influence extended into

the Renaissance (14th century - 16th century).

Aristotle's opinions were the authority on nature until

well into the 1300s. Unfortunately, many of

Aristotle's opinions were wrong. It is not intended

here to denigrate Aristotle's intelligence; he was

without doubt a brilliant man. It was simply that he

was using a method for determining the nature of the

physical world that is inadequate for that task. The

philosopher's method was logical thinking, not

making observations on the natural world. This led to

many errors in Aristotle's thinking on nature. Let's

consider two of Aristotle's opinions as examples.

In Aristotle's opinion, men were bigger and

stronger than women; therefore, it was logical to him that men would have more teeth than

women. Thus, Aristotle concluded it was a true fact that men had more teeth than women.

Apparently, it never entered his mind to actually look into the mouths of both genders and

count their teeth. Had he done so, he would have found that men and women have exactly the

same number of teeth.

In terms of physical science, Aristotle thought about dropping two balls of exactly the

same size and shape but of different masses to see which one would strike the ground first. In

his mind, it was clear that the heavier ball would fall faster than the lighter one and he

concluded that this was a law of nature. Once again, he did not consider doing an experiment

to see which ball fell faster. It was logical to him, and in fact, it still seems logical. If

someone told you that the heavier ball would fall faster, you would have no reason to

disbelieve it. In fact, it is not true and the best way to prove this is to try it.

Eighteen centuries later, Galileo decided to actually get two balls of different masses,

but with the same size and shape, and drop them off a building (Legend says the Leaning

Tower of Pisa), and actually see which one hit the ground first. When Galileo actually did the

experiment, he discovered, by observation, that the two balls hit the gScientific Methods of Problem Solving

In the 16th and 17th centuries, innovative thinkers were developing a new way to

discover the nature of the world around them. They were developing a method that relied upon

making observations of phenomena and insisting that their explanations of the nature of the

phenomena corresponded to the observations they made.

The scientific method is a method of investigation involving experimentation and

observation to acquire new knowledge, solve problems, and answer questions. Scientists

frequently list the scientific method as a series of steps. Other scientists oppose this listing of

steps because not all steps occur in every case, and sometimes the steps are out of order. The

scientific method is listed in a series of steps here because it makes it easier to study. You should

remember that not all steps occur in every case, nor do they always occur in order.

The Steps in the Scientific Method

Step 1: Identify the problem or

phenomenon that needs explaining. This

is sometimes referred to as "defining the

problem."

Step 2: Gather and organize data on the

problem. This step is also known as

"making observations."

Step 3: Suggest a possible solution or

explanation. A suggested solution is

called a hypothesis.

Step 4: Test the hypothesis by making

new observations.

Step 5: If the new observations support

the hypothesis, you accept the hypothesis

for further testing. If the new

observations do not agree with your

hypothesis, add the new observations to

your observation list and return to Step 3.

Experimentation

Experimentation is the primary way through which science gathers evidence for

ideas. It is more successful for us to cause something to happen at a time and place of our

choosing. When we arrange for the phenomenon to occur at our convenience, we can have all

our measuring instruments present and handy to help us make observations, and we can

control other variables. Experimentation involves causing a phenomenon to occur when and

where we want it and under the conditions we want. An experiment is a controlled method

of testing an idea or to find patterns. When scientists conduct experiments, they are usually

seeking new information or trying to verify someone else's data.

Experimentation involves changing and looking at many variables. The independent

variable is the part of the experiment that is being changed or manipulated. There can only

be one independent variable in any experiment. Consider, for example, that you were trying

to determine the best fertilizer for your plants. It would be important for you to grow your

plants with everything else about how they are grown being the same except fyou were using. You would be changing the type of fertilizer you gave the plants and this

would be the independent variable. If you also changed how much water the plants received,

the type of plants you were growing, and some of the plants were grown inside and others

outside, you could not determine whether or not it was actually the fertilizer that caused the

plants to grow better or if it was something else you had changed. This is why it is important

that there is only one independent variable.

The dependent variable is what is observed or measured as a result of what

happened when the independent variable was changed. In the plant experiment described

above, you might measure the height of the plant and record their appearance and color.

These would be the dependent variables. The dependent variable is also sometimes called

the resultant variable.

Controlled variables are conditions of the experiment that are kept the same for

various trials of the experiment. Once again, if we were testing how fertilizer affected how

well our plants grew, we would want everything else about how the plants are grown to be

kept the same. We would need to use the same type of plant (maybe green beans), give them

the same amount of water, plant them in the same location (all outside in the garden), give

them all the same pesticide treatment, etc. These would be controlled variables.

Suppose a scientist, while walking along the beach on a very cold day following a

rainstorm, observed two pools of water in bowl shaped rocks near each other. One of the

pools was partially covered with ice, while the other pool had no ice on it. The unfrozen pool

seemed to be formed from seawater splashing up on the rock from the surf, but the other pool

was too high for seawater to splash in, so it was more likely to have been formed from

rainwater.

The scientist wondered why one pool was partially frozen and not the other, since

both pools were at the same temperature. By tasting the water (not a good idea), the scientist

determined that the unfrozen pool tasted saltier than the partially frozen one. The scientist

thought perhaps salt water had a lower freezing point than fresh water, and she decided to go

home and try an experiment to see if this were true. So far, the scientist has identified a

question, gathered a small amount of data, and suggested an explanation. In order to test this

hypothesis, the scientist will conduct an experiment during which she can make accurate

observations.

For the experiment, the scientist prepared two identical

containers of fresh water and added some salt to one of them.

A thermometer was placed in each liquid and these were put in

a freezer. The scientist then observed the conditions and

temperatures of the two liquids at regular intervals.

The Temperature and Condition of Fresh

Water in a Freezer

Time (min) Temp (°C) Condition

0 25 Liquid

5 20 Liquid

10 15 Liquid

15 10 Liquid

20 5 Liquid

25 0 Frozen

30 -5 Frozen

The Tpossible. A model may be as uncomplicated as a sphere representing the earth or billiard

balls representing gaseous molecules, or as complex as mathematical equations representing

light.

Chemists rely on both careful observation and well-known physical laws. By putting

observations and laws together, chemists develop models. Models are really just ways of

predicting what will happen given a certain set of circumstances. Sometimes these models

are mathematical, but other times, they are purely descriptive.

If you were asked to determine the contents of a box that cannot be opened, you

would do a variety of experiments in order to develop an idea (or a model) of what the box

contains. You would probably shake the box, perhaps put magnets near it and/or determine

its mass. When you completed your experiments, you would develop an idea of what is

inside; that is, you would make a model of what is inside a box that cannot be opened.

A good example of how a model is useful to scientists is how models were used to

explain the development of the atomic theory. As you will learn in a later chapter, the idea of

the concept of an atom changed over many years. In order to understand each of the different

theories of the atom according to the various scientists, models were drawn, and the concepts

were more easily understood.

Chemists make up models about what happens when different chemicals are mixed

together, or heated up, or cooled down, or compressed. Chemists invent these models using

many observations from experiments in the past, and they use these models to predict what

might happen during experiments in the future. Once chemists have models that predict the

outcome of experiments reasonably well, those working models can help to tell them what

they need to do to achieve a certain desired result. That result might be the production of an

especially strong plastic, or it might be the detection of a toxin when it's present in your

food.

Lesson Summary

A hypothesis is a tentative explanation that can be tested by further investigation.

A theory is a well-supported explanation of observations.

A scientific law is a statement that summarizes the relationship between variables.

An experiment is a controlled method of testing a hypothesis.

A model is a description, graphic, or 3-D representation of theory used to help

enhance understanding.

Scientists often use models when they need a way to communicate their

understanding of what might be very small (such as an atom or molecule) or very

large (such as the universe).

Vocabulary

Hypothesis: A tentative explanation that can be tested by further investigation.

Theory: A well-established explanation

Scientific law: A statement that summarizes the relationship between variables.

Model: A description, graphic, or 3-D representation of theory used to help enhance

understanding.

Further Reading / Supplemental Links

http://en.wikipedia.org/wiki/Scientific_theory

17

www.ck12.org

http://en.wikipedia.org/wiki/Hypothesis

Video on Demand – Modeling the Unseen

(http://www.learner.org/resources/series61.html?pop=yes&pid=793#)

1.2: Review Questions

Multiple Choice

1) A number of people became ill after eating oysters in a restaurant. Which of the

following statements is a hypothesis about this occurrence?

a) Everyone who ate oysters got sick.

b) People got sick whether the oysters they ate were raw or cooked.

c) Symptoms included nausea and dizziness.

d) Bacteria in the oysters may have caused the illness.

2) If the hypothesis is rejected (proved wrong) by the experiment, then:

a) The experiment may have been a success.

b) The experiment was a failure.

c) The experiment was poorly designed.

d) The experiment didn't follow the scientific method.

3) A hypothesis is:

a) A description of a consistent pattern in observations.

b) An observation that remains constant.

c) A theory that has been proven.

d) A tentative explanation for a phenomenon.

4) A scientific law is:

a) A description of a consistent pattern in observations.

b) An observation that remains constant.

c) A theory that has been proven.

d) A tentative explanation for a phenomenon.

5) A well-substantiated explanation of an aspect of the natural world is a:

a) Theory.

b) Law.

c) Hypothesis.

d) None of these.

6) Which of the following words is closest to the same meaning as hypothesis?

a) Fact

b) Law

c) Formula

d) Suggestion

e) Conclusion

7) Why do scientists sometimes discard theories?

a) The steps in the scientific method were not followed in order.

b) Public opinion disagrees with the theory.

c) The theory is opposed by the church.

d) Contradictory observations are found.

8) True/False: When a theory has been known for a long time, it becomes a law.

18

www.ck12.org

1.3: Graphing

Objectives

Correctly graph data utilizing dependent variable, independent variable, scale and

units of a graph, and best fit curve.

Recognize patterns in data from a graph.

Solve for the slope of given line graphs.

Introduction

Scientists search for regularities and trends in

data. Two common methods of presenting data that aid

in the search for regularities and trends are tables and

graphs. The table below presents data about the pressure

and volume of a sample of gas. You should note that all

tables have a title and include the units of the

measurements.

You may note a regularity that appears in this

table; as the volume of the gas decreases (gets

smaller), its pressure increases (gets bigger). This

regularity or trend becomes even more apparent in a

graph of this data. A graph is a pictorial

representation of patterns using a coordinate system.

When the data from the table is plotted as a graph, the

trend in the relationship between the pressure and

volume of a gas sample becomes more apparent. The

graph gives the scientist information to aid in the

search for the exact regularity that exists in these data.

When scientists record their results in a data table, the independent variable is put in

the first column(s), the dependent variable is recorded in the last column(s) and the

controlled variables are typically not included at all. Note in the data table that the first

column is labeled "Volume (in liters)" and that the second column is labeled "Pressure (in

atm). That indicates that the volume was being changed (the independent variable) to see

how it affected the pressure (dependent variable).

In a graph, the independent variable is recorded along the x-axis (horizontal axis) or

as part of a key for the graph, the dependent variable is recorded along the y-axis (vertical

axis), and the controlled variables are not included at all. Note in the data table that the Xaxis is labeled "Volume (liters)" and that the Y-axis is labeled "Pressure (atm). That

indicates that the volume was being changed (the independent variable) to see how it affected

the pressure (dependent variable).

Drawing Line Graphs

Reading information from a line graph is easier and more accurate as the size of the

graph increases. In the two graphs shown below, the first graph uses only a small fraction of

the space available on the graph paper. The second graph uses all the space available for the

same graph. If you were attempting to determine the pressure at a temperature of 260 K,

using the graph on the left would give a less accurate result than using the graph on the right.

Volume

(liters)

Pressure

(atm)

10.0 0.50

5.0 1.00

3.33 1.50

2.50 2.00

2.00 2.50

1.67 3.00

CC – Tracy Poulsen

19

www.ck12.org

When you draw a line graph, you

should arrange the numbers on the axis to

use as much of the graph paper as you can.

If the lowest temperature in your data is

100 K and the highest temperature in your

data is 160 K, you should arrange for 100

K to be on the extreme left of your graph

and 160 K to be on the extreme right of

your graph. The creator of the graph on the

left did not take this advice and did not

produce a very good graph. You should

also make sure that the axis on your graph

are labeled and that your graph has a title.

When constructing a graph, there are some

general principles to keep in mind:

Take up as much of the graph paper

as possible. The lowest x-value

should be on the far left of the paper

and the highest x-value should be

on the far right side of the paper.

Your lowest y-value should be near

the bottom of the graph and the

highest y-value near the top.

Choose your scale to allow you to

do this. You do not need to start

counting at zero.

Count your x- and y-scales by consistent amounts. If you start counting your x-axis

where every box counts as 2-units, you must count that way the course of the entire

axis. Your y-axis may count by a different scale (maybe every box counts as 5

instead), but you must count the entire y-axis by that scale.

Both of your axis should be labeled, including units. What was measured along that

axis and what unit was it measured in?

For X-Y scatter plots, draw a best-fit-line or curve that fits your data, instead of

connecting the dots. You want a line that shows the overall trend in the data, but

might not hit exactly all of your data points. What is the overall pattern in the data?

Reading Information from a Graph

When we draw a line graph from a set of data points, we are creating data points

between known data points. This process is called interpolation. Even though we may have

four actual data points that were measured, we assume the relationship that exists between

the quantities at the actual data points also exists at all the points on the line graph between

the actual data points. Consider the following set of data for the solubility of KClO3 in water.

The table shows that there are exactly six known data points. When the data is

graphed, however, the graph maker assumes that the relationship between the temperature

CC – Tracy Poulsen

20

www.ck12.org

and the solubility remains the same. The line is drawn by interpolating the data points

between the actual data points.

We can now reasonably certainly read data from the graph for points that were not

actually measured. If we wish to determine the solubility of KClO3 at 70°C, we follow the

vertical grid line for 70°C up to where it touches the graphed line and then follow the

horizontal grid line to the axis to read the solubility. In this case, we would read the solubility

to be 30. g/100 mL of H2O at 70°C.

There are also occasions when scientists

wish to determine data points from a graph that are

not between actual data points but are beyond the

ends of the actual data points. Creating data points

beyond the end of the graph line, using the basic

shape of the curve as a guide is called

extrapolation.

Suppose the graph for the solubility of

potassium chlorate has been made from just three

actual data points. If the actual data points for the

curve were the solubility at 60°C, 80°C, and

100°C, the graph would be the solid line shown on

the graph above. If the solubility at 30°C was desired, we could extrapolate (the dotted line)

from the graph and suggest the solubility to be 5.0 g/100 mL of H2O. If we check on the

more complete graph above, you can see that the solubility at 30°C is close to 10 g/100 mL

of H2O. The reason the second graph produces such a poor answer is that the relationship that

appears in the less complete graph does not hold beyond the ends of the graph. For this

reason, extrapolation is only acceptable for graphs where there is evidence that the

relationship shown in the graph will be true beyond the ends of the graph. Extrapolation is

more dangerous that interpolation in terms of possibly producing incorrect data.

In situations in which both the independent and dependent variables are measured or

counted quantities, an X-Y scatter plot is the most useful and appropriate type of graph. A

line graph cannot be used for independent variables that are groups of data, or nonmeasured

data. In these situations in which groups of data, rather than exact measurements, were

recorded as the independent variable, a bar graph can typically be used. Consider the data in

the following table.

For this data, a bar graph is more appropriate because independent variable is a group, 2.1: Early Ideas of Atoms

Objectives

Give a short history of the concept of the atom.

Describe the contributions of Democritus and Dalton to atomic theory.

Summarize Dalton's atomic theory and explain its historical development.

Introduction

You learned earlier how all matter in the universe is made out of tiny building blocks

called atoms. All modern scientists accept the concept of the atom, but when the concept of

the atom was first proposed about 2,500 years ago, ancient philosophers laughed at the idea.

It has always been difficult to convince people of the existence of things that are too small to

see. We will spend some time considering the evidence (observations) that convince

scientists of the existence of atoms.

Democritus and the Greek Philosophers

Before we discuss the experiments and evidence

that have, over the years, convinced scientists that matter is

made up of atoms, it's only fair to give credit to the man

who proposed "atoms" in the first place. About 2,500 years

ago, early Greek philosophers believed the entire universe

was a single, huge, entity. In other words, "everything was

one." They believed that all objects, all matter, and all

substances were connected as a single, big, unchangeable

"thing."

One of the first people to propose "atoms" was a

man known as Democritus. As an alternative to the beliefs

of the Greek philosophers, he suggested that atomos, or

atomon – tiny, indivisible, solid objects - make up all

matter in the universe.

Democritus then reasoned that changes occur when

the many atomos in an object were reconnected or

recombined in different ways. Democritus even extended

his theory, suggesting that there were different varieties of

atomos with different shapes, sizes, and masses. He

thought, however, that shape, size and mass were the only properties differentiating the

different types of atomos. According to Democritus, other characteristics, like color and

taste, did not reflect properties of the atomos themselves, but rather, resulted from the

different ways in which the atomos were combined and connected to one another.

Greek philosophers truly believed that, above all else, our understanding of the world

should rely on "logic." In fact, they argued that the world couldn't be understood using our

senses at all, because our senses could deceive us. Therefore, instead of relying on

observation, Greek philosophers tried to understand the world using their minds and, more

specifically, the power of reason.

Democritus was known as "The

Laughing Philosopher." It's a good

thing he liked to laugh, because most

other philosophers were laughing at

his theories.

25

www.ck12.org

So how could the Greek philosophers have known that

Democritus had a good idea with his theory of "atomos?" It

would have taken some careful observation and a few simple

experiments. Now you might wonder why Greek philosophers

didn't perform any experiments to actually test Democritus'

theory. The problem, of course, was that Greek philosophers

didn't believe in experiments at all. Remember, Greek

philosophers didn't trust their senses, they only trusted the

reasoning power of the mind.

The early Greek philosophers tried to understand the

nature of the world through reason and logic, but not through

experiment and observation. As a result, they had some very

interesting ideas, but they felt no need to justify their ideas

based on life experiences. In a lot of ways, you can think of the

Greek philosophers as being "all thought and no action." It's

truly amazing how much they achieved using their minds, but

because they never performed any experiments, they missed or

rejected a lot of discoveries that they could have made otherwise. Greek philosophers

dismissed Democritus' theory entirely. Sadly, it took over two millennia before the theory of

atomos (or "atoms," as they're known today) was fully appreciated.

Dalton's Atomic Theory

Although the concept of atoms is now widely accepted, this wasn't always the case.

Scientists didn't always believe that everything was composed of small particles called

atoms. The work of several scientists and their experimental data gave evidence for what is

now called the atomic theory.

In the late 1700's, Antoine Lavoisier, a French scientist, experimented with the

reactions of many metals. He carefully measured the mass of a substance before reacting and

again measured the mass after a reaction had occurred in a closed system (meaning that

nothing could enter or leave the container). He found that no matter what reaction he looked

at, the mass of the starting materials was always equal to the mass of the ending materials.

This is now called the law of conservation of mass. This went contrary to what many

scientists at the time thought. For example, when a piece of iron rusts, it appears to gain

mass. When a log is burned, it appears to lose mass. In these examples, though, the reaction

does not take place in a closed container and substances, such as the gases in the air, are able

to enter or leave. When iron rusts, it is combining with oxygen in the air, which is why it

seems to gain mass. What Lavoisier found was that no mass was actually being gained or

lost. It was coming from the air. This was a very important first step in giving evidence for

the idea that everything is made of atoms. The atoms (and mass) are not being created or

destroyed. The atoms are simply reacting with other atoms that are already present.

In the late 1700s and early 1800s, scientists began noticing that when certain

substances, like hydrogen and oxygen, were combined to produce a new substance, like

water, the reactants (hydrogen and oxygen) always reacted in the same proportions by mass.

In other words, if 1 gram of hydrogen reacted with 8 grams of oxygen, then 2 grams of

hydrogen would react with 16 grams of oxygen, and 3 grams of hydrogen would react with

24 grams of oxygen. Strangely, the observation that hydrogen and oxygen always reacted in

Greek philosophers tried to

understand the nature of the

world through reason and

logic but not through

experiment and observation.

26

www.ck12.org

the "same proportions by mass" wasn't special. In fact, it turned out that the reactants in

every chemical reaction reacted in the same proportions by mass. This observation is

summarized in the law of definite proportions. Take, for example, nitrogen and hydrogen,

which react to produce ammonia. In chemical reactions, 1 gram of hydrogen will react with

4.7 grams of nitrogen, and 2 grams of hydrogen will react with 9.4 grams of nitrogen. Can

you guess how much nitrogen would react with 3 grams of hydrogen? Scientists studied

reaction after reaction, but every time the result was the same. The reactants always reacted

in the same proportions.

At the same time that scientists were finding this pattern

out, a man named John Dalton was experimenting with several

reactions in which the reactant elements formed more than one

type of product, depending on the experimental conditions he

used. One common reaction that he studied was the reaction

between carbon and oxygen. When carbon and oxygen react,

they produce two different substances – we'll call these

substances "A" and "B." It turned out that, given the same

amount of carbon, forming B always required exactly twice as

much oxygen as forming A. In other words, if you can make A

with 3 grams of carbon and 4 grams of oxygen, B can be made

with the same 3 grams of carbon, but with 8 grams oxygen.

Dalton asked himself – why does B require 2 times as much

oxygen as A? Why not 1.21 times as much oxygen, or 0.95

times as much oxygen? Why a whole number like 2?

The situation became even stranger when Dalton tried

similar experiments with different substances. For example,

when he reacted nitrogen and oxygen, Dalton discovered that he could make three different

substances – we'll call them "C," "D," and "E." As it turned out, for the same amount of

nitrogen, D always required twice as much oxygen as C. Similarly, E always required exactly

four times as much oxygen as C. Once again, Dalton noticed that small whole numbers (2

and 4) seemed to be the rule. This observation came to be known as the law of multiple

proportions.

Dalton thought about his results and tried to find some theory that would explain it, as

well as a theory that would explain the Law of Conservation of Mass (mass is neither created

nor destroyed, or the mass you have at the beginning is equal to the mass at the end of a

change). One way to explain the relationships that Dalton and others had observed was to

suggest that materials like nitrogen, carbon and oxygen were composed of small, indivisible

quantities which Dalton called "atoms" (in reference to Democritus' original idea). Dalton

used this idea to generate what is now known as Dalton's Atomic Theory which stated the

following:

1. Matter is made of tiny particles called atoms.

2. Atoms are indivisible (can't be broken into smaller particles). During a chemical

reaction, atoms are rearranged, but they do not break apart, nor are they created or

destroyed.

3. All atoms of a given element are identical in mass and other properties.

4. The atoms of different elements differ in mass and other properties.

Unlike the Greek

philosophers, John Dalton

believed in both logical

thinking and

experimentation.

27

www.ck12.org

5. Atoms of one element can combine with atoms of another element to form

"compounds" – new, complex particles. In a given compound, however, the

different types of atoms are always present in the same relative numbers.

Lesson Summary

2,500 years ago, Democritus suggested that all matter in the universe was made up of

tiny, indivisible, solid objects he called "atomos."

Other Greek philosophers disliked Democritus' "atomos" theory because they felt it

was illogical.

Dalton used observations about the ratios in which elements will react to combine and

The Law of Conservation of Mass to propose his Atomic Theory.

Dalton's Atomic Theory states:

1. Matter is made of tiny particles called atoms.

2. Atoms are indivisible. During a chemical reaction, atoms are rearranged, but they

do not break apart, nor are they created or destroyed.

3. All atoms of a given element are identical in mass and other properties.

4. The atoms of different elements differ in mass and other properties.

5. Atoms of one element can combine with atoms of another element to form

"compounds" – new complex particles. In a given compound, however, the different

types of atoms are always present in the same relative numbers.

Vocabulary

Atom: Democritus' word for the tiny, indivisible, solid objects that he believed made

up all matter in the universe

Dalton's Atomic Theory: the first scientific theory to relate chemical changes to the

structure, properties, and behavior of the atom

Further Reading / Supplemental Links

To see a video documenting the early history of the concept of the atom, go to

http://www.uen.org/dms/. Go to the k-12 library. Search for "history of the atom".

Watch part 01. (you can get the username and password from your teacher)

Vision Learning: From Democritus to Dalton:

http://visionlearning.com/library/module_viewer.php?c3=&mid=49&l=

2.1: Review Questions

1) (Multiple choice) Which of the following is not part of Dalton's Atomic Theory?

a) matter is made of tiny particles called atoms.

b) during a chemical reaction, atoms are rearranged.

c) during a nuclear reaction, atoms are split apart.

d) all atoms of a specific element are the same.

2) Democritus and Dalton both suggested that all matter was composed of small particles,

called atoms. What is the greatest advantage Dalton's Atomic Theory had over

Democritus'?

28

www.ck12.org

3) It turns out that a few of the ideas in Dalton's Atomic Theory aren't entirely correct. Are

inaccurate theories an indication that science is a waste of time?

2.2: Further Understanding of the Atom

Objectives

Explain the observations that led to Thomson's discovery of the electron.

Describe Thomson's "plum pudding" mode of the atom and the evidence for it

Draw a diagram of Thomson's "plum pudding" model of the atom and explain why it

has this name.

Describe Rutherford's gold foil experiment and explain how this experiment altered

the "plum pudding" model.

Draw a diagram of the Rutherford model of the atom and label the nucleus and the

electron cloud.

Introduction

Dalton's Atomic Theory held up well to a lot of the

different chemical experiments that scientists performed to test

it. In fact, for almost 100 years, it seemed as if Dalton's

Atomic Theory was the whole truth. However, in 1897, a

scientist named J. J. Thomson conducted some research that

suggested that Dalton's Atomic Theory wasn't the entire story.

As it turns out, Dalton had a lot right. He was right in saying

matter is made up of atoms; he was right in saying there are

different kinds of atoms with different mass and other

properties; he was "almost" right in saying atoms of a given

element are identical; he was right in saying during a chemical

reaction, atoms are merely rearranged; he was right in saying a

given compound always has atoms present in the same relative

numbers. But he was WRONG in saying atoms were

indivisible or indestructible. As it turns out, atoms are

divisible. In fact, atoms are composed of even smaller, more fundamental particles. These

particles, called subatomic particles, are particles that are smaller than the atom. We'll talk

about the discoveries of these subatomic particles next.

Thomson's Plum Pudding Model

In the mid-1800s, scientists were beginning to realize that the study of chemistry and

the study of electricity were actually related. First, a man named Michael Faraday showed

how passing electricity through mixtures of different chemicals could cause chemical

reactions. Shortly after that, scientists found that by forcing electricity through a tube filled

with gas, the electricity made the gas glow! Scientists didn't, however, understand the

relationship between chemicals and electricity until a British physicist named J. J. Thomson

began experimenting with what is known as a cathode ray tube.

The figure shows a basic diagram of a cathode ray tube like the one J. J. Thomson

would have used. A cathode ray tube is a small glass tube with a cathode (a negatively

charged metal plate) and an anode (a positively charged metal plate) at opposite ends. By

J.J. Thomson conducted

experiments that suggested

that Dalton's atomic theory

wasn't telling the entire

story.

29

www.ck12.org

separating the cathode and anode by a short distance, the cathode ray tube can generate what

are known as cathode rays – rays of electricity that flow from the cathode to the anode. J. J.

Thomson wanted to know what cathode rays were, where cathode rays came from, and

whether cathode rays had any mass or charge. The techniques that J. J. Thomson used to

answer these questions were very clever and earned him a Nobel Prize in physics. First, by

cutting a small hole in the anode, J. J. Thomson found that he could get some of the cathode

rays to flow through the hole in the anode and into the other end of the glass cathode ray

tube. Next, J. J. Thomson figured out that if he painted a substance known as "phosphor"

onto the far end of the cathode ray tube, he could see exactly where the cathode rays hit

because the cathode rays made the phosphor glow.

J. J.

Thomson must have

suspected that

cathode rays were

charged, because

his next step was to

place a positively

charged metal plate

on one side of the

cathode ray tube

and a negatively

charged metal plate

on the other side of

the cathode ray

tube, as shown in

Figure 3. The metal

plates didn't actually touch the cathode ray tube, but they were close enough that a

remarkable thing happened! The flow of the cathode rays passing through the hole in the

anode was bent upwards towards the positive metal plate and away from the negative metal

plate. Using the "opposite charges attract, like charges repel" rule, J. J. Thomson argued that

if the cathode rays were attracted to the positively charged metal plate and repelled from the

negatively charged metal plate, they themselves must have a negative charge!

J. J. Thomson then did some rather complex experiments with magnets, and used his

results to prove that cathode rays were not only negatively charged, but also had mass.

Remember that anything with mass is part of what we call matter. In other words, these

cathode rays must be the result of negatively charged "matter" flowing from the cathode to

the anode. But there was a problem. According to J. J. Thomson's measurements, either these

cathode rays had a ridiculously high charge, or else had very, very little mass – much less

mass than the smallest known atom. How was this possible? How could the matter making

up cathode rays be smaller than an atom if atoms were indivisible? J. J. Thomson made a

radical proposal: maybe atoms are divisible. J. J. Thomson suggested that the small,

negatively charged particles making up the cathode ray were actually pieces of atoms. He

called these pieces "corpuscles," although today we know them as electrons. Thanks to his

clever experiments and careful reasoning, J. J. Thomson is credited with the discovery of the

electron.

Thomson's experiment with cathode rays found that the ray moved away

from negatively charged plates and toward positively charges plates. What

does this say about the charge of the ray?

CC – Tracy Poulsen

30

www.ck12.org

Now imagine what would happen if atoms were made entirely of electrons. First of

all, electrons are very, very small; in fact, electrons are about 2,000 times smaller than the

smallest known atom, so every atom would have to contain a whole lot of electrons. But

there's another, even bigger problem: electrons are negatively charged. Therefore, if atoms

were made entirely out of electrons, atoms would be negatively charged themselves… and

that would mean all matter was negatively charged as well. Of course, matter isn't negatively

charged. In fact, most matter is what we call neutral – it has no charge at all. If matter is

composed of atoms, and atoms are composed of negative electrons, how can matter be

neutral? The only possible explanation is that atoms consist of more than just electrons.

Atoms must also contain some type of positively charged material that balances the negative

charge on the electrons. Negative and positive charges of equal size cancel each other out,

just like negative and positive numbers of equal size. What do you get if you add +1 and -1?

You get 0, or nothing. That's true of numbers, and that's also true of charges. If an atom

contains an electron with a -1 charge, but also some form of material with a +1 charge,

overall the atom must have a (+1) + (-1) = 0 charge – in other words, the atom must be

neutral, or have no charge at all.

Based on the fact that atoms are neutral, and based on J. J. Thomson's discovery that

atoms contain negative subatomic particles called "electrons," scientists assumed that atoms

must also contain a positive substance. It turned out that this positive substance was another

kind of subatomic particle, known as the proton. Although scientists knew that atoms had to

contain positive material, protons weren't actually discovered, or understood, until quite a bit

later.

When Thomson discovered the negative electron, he realized that atoms had to

contain positive material as well – otherwise they wouldn't be neutral overall. As a result,

Thomson formulated what's known as the "plum pudding" model for the atom. According to

the "plum pudding" model, the negative electrons were

like pieces of fruit and the positive material was like the

batter or the pudding. This made a lot of sense given

Thomson's experiments and observations. Thomson had

been able to isolate electrons using a cathode ray tube;

however he had never managed to isolate positive

particles. As a result, Thomson theorized that the positive

material in the atom must form something like the "batter"

in a plum pudding, while the negative electrons must be

scattered through this "batter." (If you've never seen or

tasted a plum pudding, you can think of a chocolate chip

cookie instead. In that case, the positive material in the

atom would be the "batter" in the chocolate chip cookie,

while the negative electrons would be scattered through

the batter like chocolate chips.)

Notice how easy it would be to pick the pieces of fruit out of a plum pudding. On the

other hand, it would be a lot harder to pick the batter out of the plum pudding, because the

batter is everywhere. If an atom were similar to a plum pudding in which the electrons are

scattered throughout the "batter" of positive material, then you'd expect it would be easy to

pick out the electrons, but a lot harder to pick out the positive material.

Thomson's plum pudding model

was much like a chocolate chip

cookie. Notice how the chocolate

chips are the negatively charged

electrons, while the positive charge

is spread throughout the entire

31

www.ck12.org

J.J. Thomson had measured the charge to mass ratio of the electron, but had been

unable to accurately measure the charge on the electron. With his oil drop experiment, Robert

Millikan was able to accurately measure the charge of the electron. When combined with the

charge to mass ratio, he was able to calculate the mass of the electron. What Millikan did was

to put a charge on tiny droplets of oil and measured their rate of descent. By varying the

charge on different drops, he noticed that the electric charges on the drops were all multiples

of 1.6x10-19C, the charge on a single electron.

Rutherford's Nuclear Model

Everything about Thomson's experiments suggested

the "plum pudding" model was correct – but according to the

scientific method, any new theory or model should be tested

by further experimentation and observation. In the case of the

"plum pudding" model, it would take a man named Ernest

Rutherford to prove it inaccurate. Rutherford and his

experiments will be the topic of the next section.

Disproving Thomson's "plum pudding" model began

with the discovery that an element known as uranium emits

positively charged particles called alpha particles as it

undergoes radioactive decay. Radioactive decay occurs when

one element decomposes into another element. It only happens

with a few very unstable elements. Alpha particles themselves

didn't prove anything about the structure of the atom, they

were, however, used to conduct some very interesting experiments.

Ernest Rutherford was fascinated by all aspects of alpha particles. For the most part,

though, he seemed to view alpha particles as tiny bullets that he could use to fire at all kinds

of different materials. One experiment in particular, however, surprised Rutherford, and

everyone else.

Rutherford found that

when he fired alpha particles

at a very thin piece of gold

foil, an interesting thing

happened. Almost all of the

alpha particles went straight

through the foil as if they'd

hit nothing at all. This was

what he expected to happen.

If Thomson's model was

accurate, there was nothing

hard enough for these small

particles to hit that would

cause any change in their

motion.

Every so often,

though, one of the alpha

particles would be deflected

Ernest Rutherford

Ernest Rutherford's Gold Foil Experiment in which alpha particles were

shot at a piece of gold foil. Most of the particles went straight through, but

some bounced straight back, indicating they were hitting a very small, very

dense particle in the atom.

CC – Tracy Poulsen

32

www.ck12.org

slightly as if it had bounced off of something hard. Even less often, Rutherford observed

alpha particles bouncing straight back at the "gun" from which they had been fired! It was as

if these alpha particles had hit a wall "head-on" and had ricocheted right back in the direction

that they had come from.

Rutherford thought that these experimental results were rather odd. Rutherford

described firing alpha particles at gold foil like shooting a high-powered rifle at tissue paper.

Would you ever expect the bullets to hit the tissue paper and bounce back at you? Of course

not! The bullets would break through the tissue paper and keep on going, almost as if they'd

hit nothing at all. That's what Rutherford had expected would happen when he fired alpha

particles at the gold foil. Therefore, the fact that most alpha particles passed through didn't

shock him. On the other hand, how could he explain the alpha particles that got deflected?

Furthermore, how could he explain the alpha particles that bounced right back as if they'd hit

a wall?

Rutherford decided that the only way to explain his results was to assume that the

positive matter forming the gold atoms was not, in fact, distributed like the batter in plum

pudding, but rather, was concentrated in one spot, forming a small positively charged particle

somewhere in the center of the gold atom. We now call this clump of positively charged

mass the nucleus. According to Rutherford, the presence of a nucleus explained his

experiments, because it implied that most alpha particles passed through the gold foil without

hitting anything at all. Once in a while, though, the alpha particles would actually collide

with a gold nucleus, causing the alpha particles to be deflected, or even to bounce right back

in the direction they came from.

While Rutherford's discovery of the positively

charged atomic nucleus offered insight into the structure of

the atom, it also led to some questions. According to the

"plum pudding" model, electrons were like plums embedded

in the positive "batter" of the atom. Rutherford's model,

though, suggested that the positive charge wasn't distributed

like batter, but rather, was concentrated into a tiny particle at

the center of the atom, while most of the rest of the atom was

empty space. What did that mean for the electrons? If they

weren't embedded in the positive material, exactly what were

they doing? And how were they held in the atom? Rutherford

suggested that the electrons might be circling or "orbiting"

the positively charged nucleus as some type of negatively

charged cloud, but at the time, there wasn't much evidence to

suggest exactly how the electrons were held in the atom.

Despite the problems and questions associated with

Rutherford's experiments, his work with alpha particles definitely seemed to point to the

existence of an atomic "nucleus." Between J. J. Thomson, who discovered the electron, and

Rutherford, who suggested that the positive charges in an atom were concentrated at the

atom's center, the 1890s and early 1900s saw huge steps in understanding the atom at the

"subatomic" (or smaller than the size of an atom) level. Although there was still some

uncertainty with respect to exactly how subatomic particles were organized in the atom, it

was becoming more and more obvious that atoms were indeed divisible. Moreover, it was

clear that an atom contains negatively charged electrons and a nucleus containing positive

Rutherford suggested that

electrons surround a

central nucleus.

(Obtained from:

http://upload.wikimedia.org/wiki

pedia/commons/7/7d/Rutherford

sches_Atommodell.png)

33

www.ck12.org

charges. In the next section, we'll look more carefully at the structure of the nucleus, and

we'll learn that while the atom is made up of positive and negative particles, it also contains

neutral particles that neither Thomson, nor Rutherford, were able to detect with their

experiments.

Lesson Summary

Dalton's Atomic Theory wasn't entirely correct. It turns out that atoms can be divided

into smaller subatomic particles.

According to Thomson's "plum pudding" model, the negatively charged electrons in

an atom are like the pieces of fruit in a plum pudding, while the positively charged

material is like the batter.

When Ernest Rutherford fired alpha particles at a thin gold foil, most alpha particles

went straight through; however, a few were scattered at different angles, and some

even bounced straight back.

In order to explain the results of his Gold Foil experiment, Rutherford suggested that

the positive matter in the gold atoms was concentrated at the center of the gold atom

in what we now call the nucleus of the atom.

Vocabulary

Subatomic particles: particles that are smaller than the atom

Electron: a negatively charged subatomic particle

Proton: a positively charged subatomic particle

Nucleus: the small, dense center of the atom

Further Reading / Supplemental Material

A short history of the changes in our model of the atom, an image of the plum

pudding model, and an animation of Rutherford's experiment can be viewed at Plum

Pudding and Rutherford Page (http://www.newcastleschools.org.uk/nsn/chemistry/Radioactivity/Plub%20Pudding%20and%20Rutherford

%20Page.htm).

To see a video documenting the early history of the concept of the atom, go to

http://www.uen.org/dms/. Go to the k-12 library. Search for "history of the atom".

Watch part 02. (you can get the username and password from your teacher)

Vision Learning: The Early Days (Thomson, etc)

http://visionlearning.com/library/module_viewer.php?mid=50&l=&c3=

Discovery of Electron (YouTube):

http://www.youtube.com/watch%3Fv%3DIdTxGJjA4Jw

Thomson's Experiment: http://www.aip.org/history/electron/jjthomson.htm

Discovery of Atomic Nucleus (YouTube):

http://www.youtube.com/watch%3Fv%3DwzALbzTdnc8

Rutherford's Experiment:

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

2.2: Review Questions

Decide whether each of the following statements is true or false.

1) Electrons (cathode rays) are positively charged.

34

www.ck12.org

2) Electrons (cathode rays) can be repelled by a negatively charged metal plate.

3) J.J. Thomson is credited with the discovery of the electron.

4) The plum pudding model is the currently accepted model of the atom

#5-11: Match each conclusion regarding subatomic particles and atoms with the

observation/data that supports it.

Conclusion Observations

5) All atoms have electrons a. Most alpha particles shot at gold foil go straight

through, without any change in their direction.

6) Atoms are mostly empty

space.

b. A few alpha particles shot at gold foil bounce in the

opposite direction.

7) Electrons have a negative

charge

c. Some alpha particles (with positive charges) when

shot through gold foil bend away from the gold.

8) The nucleus is positively

charged

d. No matter which element Thomson put in a cathode

ray tube, the same negative particles with the same

properties (such as charge & mass) were ejected.

9) Atoms have a small, dense

nucleus

e. The particles ejected in Thomson's experiment bent

away from negatively charged plates, but toward

positively charged plates.

10) What is the name given to the tiny clump of positive material at the center of an atom?

11) Electrons are ______ negatively charged metals plates and ______ positively charged

metal plates.

Consider the following two paragraphs for #12-14

Scientist 1: Although atoms were once regarded as the smallest part of nature, they are

composed of even smaller particles. All atoms contain negatively charged particles,

called electrons. However, the total charge of any atom is zero. Therefore, this means

that there must also be positive charge in the atom. The electrons sit in a bed of

positively charged mass.

Scientist 2: It is true that atoms contain smaller particles. However, the electrons are not

floating in a bed of positive charge. The positive charge is located in the central part of

the atom, in a very small, dense mass, called a nucleus. The electrons are found outside

of the nucleus.

12) What is the main dispute between the two scientists' theories?

13) Another scientist was able to calculate the exact charge of an electron to be -1.6x10-19 C.

What effect does this have on the claims of Scientist 1? (Pick one answer)

a) Goes against his claim

b) Supports his claim

c) Has no effect on his claim.

14) If a positively charged particle was shot at a thin sheet of gold foil, what would the

second scientist predict to happen?

35

www.ck12.org

2.3: Protons, Neutrons, and Electrons in Atoms

Objectives

Describe the locations, charges, and masses and the three main subatomic particles.

Define atomic number.

Describe the size of the nucleus in relation to the size of the atom.

Define mass number.

Explain what isotopes are and how isotopes affect an element's atomic mass.

Determine the number of protons, neutrons, and electrons in an atom.

Introduction

Dalton's Atomic Theory explained a lot about matter, chemicals, and chemical

reactions. Nevertheless, it wasn't entirely accurate, because contrary to what Dalton believed,

atoms can, in fact, be broken apart into smaller subunits or subatomic particles. We have

been talking about the electron in great detail, but there are two other particles of interest to

use: protons and neutrons. In this section, we'll look at the atom a little more closely.

Protons, Electrons, and Neutrons

We already learned that J.J. Thomson discovered a negatively charged particle, called

the electron. Rutherford proposed that these electrons orbit a positive nucleus. In

subsequent experiments, he found that there is a smaller positively charged particle in the

nucleus which is called a proton. There is a third subatomic particle, known as a neutron.

Ernest Rutherford proposed the existence of a neutral particle, with the approximate mass of

a proton. Years later, James Chadwick proved that the nucleus of the atom contains this

neutral particle that had been proposed by Ernest Rutherford. Chadwick observed that when

beryllium is bombarded with alpha particles, it

emits an unknown radiation that has approximately

the same mass as a proton, but no electrical charge.

Chadwick was able to prove that the beryllium

emissions contained a neutral particle - Rutherford's

neutron.

As you might have already guessed from its

name, the neutron is neutral. In other words, it has

no charge whatsoever, and is therefore neither

attracted to nor repelled from other objects.

Neutrons are in every atom (with one exception),

and they're bound together with other neutrons and

protons in the atomic nucleus.

Before we move on, we must discuss how the different types of subatomic particles

interact with each other. When it comes to neutrons, the answer is obvious. Since neutrons

are neither attracted to, nor repelled from objects, they don't really interact with protons or

electrons (beyond being bound into the nucleus with the protons).

Even though electrons, protons, and neutrons are all types of subatomic particles, they

are not all the same size. When you compare the masses of electrons, protons and neutrons,

what you find is that electrons have an extremely small mass, compared to either protons or

neutrons. On the other hand, the masses of protons and neutrons are fairly similar, although

technically, the mass of a neutron is slightly larger than the mass of a proton. Because

Electrons are much smaller than

protons or neutrons. If an electron was

the mass of a penny, a proton or a

neutron would have the mass of a large

bowling ball!

36

www.ck12.org

protons and neutrons are so much

more massive than electrons, almost

all of the mass of any atom comes

from the nucleus, which contains all

of the neutrons and protons.

The table shown gives the

properties and locations of electrons,

protons, and neutrons. The third

column shows the masses of the three

subatomic particles in grams. The

second column, however, shows the masses of the three subatomic particles in "atomic mass

units". An atomic mass unit (amu) is defined as one-twelfth the mass of a carbon-12 atom.

Atomic mass units (amu) are useful, because, as you can see, the mass of a proton and the

mass of a neutron are almost exactly 1.0 in this unit system.

In addition to mass, another important property of subatomic particles is their charge.

You already know that neutrons are neutral, and thus have no charge at all. Therefore, we say

that neutrons have a charge of zero. What about electrons and protons? You know that

electrons are negatively charged and protons are positively charged, but what's amazing is

that the positive charge on a proton is exactly equal in magnitude (magnitude means

"absolute value" or "size when you ignore positive and negative signs") to the negative

charge on an electron. The third column in the table shows the charges of the three

subatomic particles. Notice that the charge on the proton and the charge on the electron have

the same magnitude.

Negative and positive charges of equal magnitude cancel each other out. This means

that the negative charge on an electron perfectly balances the positive charge on the proton.

In other words, a neutral atom must have exactly one electron for every proton. If a neutral

atom has 1 proton, it must have 1 electron. If a neutral atom has 2 protons, it must have 2

electrons. If a neutral atom has 10 protons, it must have 10 electrons. You get the idea. In

order to be neutral, an atom must have the same number of electrons and protons.

Atomic Number and Mass Number

Scientists can distinguish

between different elements by counting

the number of protons. If an atom has

only one proton, we know it's a

hydrogen atom. An atom with two

protons is always a helium atom. If

scientists count four protons in an atom,

they know it's a beryllium atom. An

atom with three protons is a lithium

atom, an atom with five protons is a

boron atom, an atom with six protons is

a carbon atom… the list goes on.

Since an atom of one element

can be distinguished from an atom of

another element by the number of

It is difficult to find qualities that are different from each

element and distinguish on element from another. Each

element, however, does have a unique number of protons.

Sulfur has 16 protons, silicon has 14 protons, and gold has

79 protons.

Sub-Atomic Particles, Properties and Location

Particle

Relative

Mass

(amu)

Electric

Charge Location

electron -1 outside the

nucleus

proton 1 +1 nucleus

neutron 1 0 nucleus

37

www.ck12.org

protons in its nucleus, scientists are always interested in this number, and how this number

differs between different elements. Therefore, scientists give this number a special name. An

element's atomic number is equal to the number of protons in the nuclei of any of its atoms.

The periodic table gives the atomic number of each element. The atomic number is a whole

number usually written above the chemical symbol of each element. The atomic number for

hydrogen is 1, because every hydrogen atom has 1 proton. The atomic number for helium is 2

because every helium atom has 2 protons. What is the atomic number of carbon?

Of course, since neutral atoms have to have one electron for every proton, an

element's atomic number also tells you how many electrons are in a neutral atom of that

element. For example, hydrogen has an atomic number of 1. This means that an atom of

hydrogen has one proton, and, if it's neutral, one electron as well. Gold, on the other hand,

has an atomic number of 79, which means that an atom of gold has 79 protons, and, if it's

neutral, and 79 electrons as well.

The mass number of an atom is the total number of protons and neutrons in its

nucleus. Why do you think that the "mass number" includes protons and neutrons, but not

electrons? You know that most of the mass of an atom is concentrated in its nucleus. The

mass of an atom depends on the number of protons and neutrons. You have already learned

that the mass of an electron is very, very small compared to the mass of either a proton or a

neutron (like the mass of a penny compared to the mass of a bowling ball). Counting the

number of protons and neutrons tells scientists about the total mass of an atom.

mass number A = (number of protons) + (number of neutrons)

An atom's mass number is a very easy to calculate provided you know the number of protons

and neutrons in an atom.

Example:

What is the mass number of an atom of helium that contains 2 neutrons?

Solution:

(number of protons) = 2 (Remember that an atom of helium always has 2 protons.)

(number of neutrons) = 2

mass number = (number of protons) + (number of neutrons)

mass number = 2 + 2 = 4

There are two main ways in which scientists frequently show the mass number of an

atom they are interested in. It is important to note that the mass number is not given on the

periodic table. These two ways include writing a nuclear symbol or by giving the name of

the element with the mass number written.

To write a nuclear symbol, the mass number is placed at the upper left (superscript)

of the chemical symbol and the atomic number is placed at the lower left (subscript) of the

symbol. The complete nuclear symbol for helium-4 is drawn below.

The following nuclear symbols are for a nickel nucleus with 31 neutrons and a uranium

nucleus with 146 neutrons.

38

www.ck12.org

In the nickel nucleus represented above, the atomic number 28 indicates the nucleus contains

28 protons, and therefore, it must contain 31neutrons in order to have a mass number of 59.

The uranium nucleus has 92 protons as do all uranium nuclei and this particular uranium

nucleus has 146 neutrons.

The other way of representing these nuclei would be Nickel-59 and Uranium-238,

where 59 and 238 are the mass numbers of the two atoms, respectively. Note that the mass

numbers (not the number of neutrons) is given to the side of the name.

Isotopes

Unlike the number of protons, which is always the same in atoms of the same

element, the number of neutrons can be different, even in atoms of the same element. Atoms

of the same element, containing the same number of protons, but different numbers of

neutrons are known as isotopes. Since the isotopes of any given element all contain the same

number of protons, they have the same atomic number (for example, the atomic number of

helium is always 2). However, since the isotopes of a given element contain different

numbers of neutrons, different isotopes have different mass numbers. The following two

examples should help to clarify this point.

Example:

a) What is the atomic number and the mass number of an isotope of lithium containing 3

neutrons. A lithium atom contains 3 protons in its nucleus.

b) What is the atomic number and the mass number of an isotope of lithium containing 4

neutrons. A lithium atom contains 3 protons in its nucleus.

Solution:

a) atomic number = (number of protons) = 3

(number of neutrons) = 3

mass number = (number of protons) + (number of neutrons)

mass number = 3 + 3 = 6

b) atomic number = (number of protons) = 3

(number of neutrons) = 4

mass number = (number of protons) + (number of neutrons)

mass number = 3 + 4 = 7

Notice that because the lithium atom always has 3 protons, the atomic number for

lithium is always 3. The mass number, however, is 6 in the isotope with 3 neutrons, and 7 in

the isotope with 4 neutrons. In nature, only certain isotopes exist. For instance, lithium exists

as an isotope with 3 neutrons, and as an isotope with 4 neutrons, but it doesn't exists as an

isotope with 2 neutrons, or as an isotope with 5 neutrons.

This whole discussion of isotopes brings us back to Dalton's Atomic Theory.

According to Dalton, atoms of a given element are identical. But if atoms of a given element

can have different numbers of neutrons, then they can have different masses as well! How

did Dalton miss this? It turns out that elements found in nature exist as constant uniform

mixtures of their naturally occurring isotopes. In other words, a piece of lithium always

contains both types of naturally occurring lithium (the type with 3 neutrons and the type wIsotopes are atoms of the same element (same number of protons) that have different

numbers of neutrons in their atomic nuclei.

Vocabulary

Neutron: a subatomic particle with no charge

Atomic mass unit (amu): a unit of mass equal to one-twelfth the mass of a carbontwelve atom

Atomic number: the number of protons in the nucleus of an atom

Mass number: the total number of protons and neutrons in the nucleus of an atom

Isotopes: atoms of the same element that have the same number of protons but

different numbers of neutrons

Further Reading / Supplemental Material

Jeopardy Game: http://www.quia.com/cb/36842.html

For a Bill Nye video on atoms, go to http://www.uen.org/dms/. Go to the k-12

library. Search for "Bill Nye atoms". (you can get the username and password from

your teacher)

2.3: Review Questions

Label each of the following statements as true or false.

1) The nucleus of an atom contains all of the protons in the atom.

2) The nucleus of an atom contains all of the electrons in the atom.

3) Neutral atoms must contain the same number of neutrons as protons.

4) Neutral atoms must contain the same number of electrons as protons.

Match the subatomic property with its description.

Sub-Atomic Particle Characteristics

5) electron a. has a charge of +1

6) neutron b. has a mass of approximately 1/1840 amu

7) proton c. is neither attracted to, nor repelled from charged objects

Indicate whether each statement is true or false.

8) An element's atomic number is equal to the number of protons in the nuclei of any of its

atoms.

9) A neutral atom with 4 protons must have 4 electrons.

10) An atom with 7 protons and 7 neutrons will have a mass number of 14.

11) An atom with 7 protons and 7 neutrons will have an atomic number of 14.

12) A neutral atom with 7 electrons and 7 neutrons will have an atomic number of 14.

Use the periodic table to find the symbol for the element with:

13) 44 electrons in a neutral atom

14) 30 protons

15) An atomic number of 36

41

www.ck12.org

In the table below, Column 1 contains data for 5 different elements. Column 2 contains data

for the same 5 elements, however different isotopes of those elements. Match the atom in the

first column to its isotope in the second column.

Original element Isotope of the same element

16) an atom with 2 protons and 1 neutron a. a C (carbon) atom with 6 neutrons

17) a Be (beryllium) atom with 5 neutrons b. an atom with 2 protons and 2

neutrons

18) an atom with an atomic number of 6 and mass

number of 13

c. an atom with an atomic number of 7

and a mass number of 15

19) an atom with 1 proton and a mass number of 1 d. an atom with an atomic number of 1

and 1 neutron

20) an atom with an atomic number of 7 and 7

neutrons

e. an atom with an atomic number of 4

and 6 neutrons

Write the nuclear symbol for each element described:

21) 32 neutrons in an atom with mass number of 58

22) An atom with 10 neutrons and 9 protons.

Indicate the number of protons, neutrons, and electrons in each of the following atoms:

23) He 4

2 24) Sodium-23 25) H

1

1

26) Iron-55 27) Cl 37

17 28) Boron-11

29) U238

92 30) Uranium-235

2.4: Atomic Mass

Objectives:

Explain what is meant by the atomic mass of an element.

Calculate the atomic mass of an element from the masses and relative percentages of

the isotopes of the element.

Introduction

In chemistry we very rarely deal with only one isotope of an element. We use a

mixture of the isotopes of an element in chemical reactions and other aspects of chemistry,

because all of the isotopes of an element react in the same manner. That means that we

rarely need to worry about the mass of a specific isotope, but instead we need to know the

average mass of the atoms of an element. Using the masses of the different isotopes and how

abundant each isotope is, we can find the average mass of the atoms of an element. The

atomic mass of an element is the weighted average mass of the atoms in a naturally

occurring sample of the element. Atomic mass is typically reported in atomic mass units.

42

www.ck12.org

Calculating Atomic Mass

You can calculate the atomic mass (or average mass) of an element provided you

know the relative abundances (the fraction of an element that is a given isotope) the

element's naturally occurring isotopes, and the masses of those different isotopes. We can

calculate this by the following equation:

Atomic mass = (%1)(mass1) + (%2)(mass2) + …

Look carefully to see how this equation is used in the following examples.

Example: Boron has two naturally occurring isotopes. In a sample of boron, 20% of the

atoms are B-10, which is an isotope of boron with 5 neutrons and a mass of 10 amu. The

other 80% of the atoms are B-11, which is an isotope of boron with 6 neutrons and a mass of

11 amu. What is the atomic mass of boron?

Solution: Boron has two isotopes. We will use the equation:

Atomic mass = (%1)(mass1) + (%2)(mass2) + …

Isotope 1: %1=0.20 (write all percentages as decimals), mass1=10

Isotope 2: %2=0.80, mass2=11

Substitute these into the equation, and we get:

Atomic mass = (0.20)(10) + (0.80)(11)

Atomic mass = 10.8 amu

The mass of an average boron atom, and thus boron's atomic mass, is 10.8 amu.

Example: Neon has three naturally occurring isotopes. In a sample of neon, 90.92% of the

atoms are Ne-20, which is an isotope of neon with 10 neutrons and a mass of 19.99 amu.

Another 0.3% of the atoms are Ne-21, which is an isotope of neon with 11 neutrons and a

mass of 20.99 amu. The final 8.85% of the atoms are Ne-22, which is an isotope of neon with

12 neutrons and a mass of 21.99 amu. What is the atomic mass of neon?

Solution:

Neon has three isotopes. We will use the equation:

Atomic mass = (%1)(mass1) + (%2)(mass2) + …

Isotope 1: %1=0.9092 (write all percentages as decimals), mass1=19.99

Isotope 2: %2=0.003, mass2=20.99

Isotope 3: %3=0.0885, mass3=21.99

Substitute these into the equation, and we get:

Atomic mass = (0.9092)(19.99) + (0.003)(20.99) + (0.0885)(21.99)

Atomic mass = 20.17 amu

The mass of an average neon atom is 20.17 amu

The periodic table gives the atomic mass of each element. The atomic mass is a

number that usually appears below the element's symbol in each square. Notice that atomic

mass of boron (symbol B) is 10.8, which is what we calculated in example 5, and the atomic

mass of neon (symbol Ne) is 20.18, which is what we calculated in example 6. Take time to

notice that not all periodic tables have the atomic number above the element's symbol and

the mass number below it. If you are ever confused, remember that the atomic number should

43

www.ck12.org

always be the smaller of the two and will be a whole number, while the atomic mass should

always be the larger of the two and will be a decimal number.

Lesson Summary

An element's atomic mass is the average mass of one atom of that element. An

element's atomic mass can be calculated provided the relative abundances of the

element's naturally occurring isotopes, and the masses of those isotopes are known.

The periodic table is a convenient way to summarize information about the different

elements. In addition to the element's symbol, most periodic tables will also contain

the element's atomic number, and element's atomic mass.

Vocabulary

Atomic mass: the weighted average of the masses of the isotopes of an element

2.4: Review Questions

1) Copper has two naturally occurring isotopes. 69.15% of copper atoms are Cu-63 and

have a mass of 62.93amu. The other 30.85% of copper atoms are Cu-65and have a mass

of 64.93amu. What is the atomic mass of copper?

2) Chlorine has two isotopes, Cl-35 and Cl-37. Their abundances are 75.53% and 24.47%

respectively. Calculate the atomic mass of chlorine.

2.5: The Nature of Light

Objectives

When given two comparative colors or areas in the electromagnetic spectrum,

identify which area has the higher wavelength, the higher frequency, and the higher

energy.

Describe the relationship between wavelength, frequency, and energy of light waves

(EMR)

Introduction

Most of us are familiar with waves, whether they are waves of water in the ocean,

waves made by wiggling the end of a rope, or waves made when a guitar string is plucked.

Light, also called electromagnetic radiation, is a special type of energy that travels as a

wave.

Light Energy

Before we talk about the different

forms of light or electromagnetic radiation

(EMR), it is important to understand some

of the general characteristics that waves

share.

The high point of a wave is called

the crest. The low point is called the

trough. The distance from one point on a

wave to the same point on the next wave is called the wavelength of the wave. You could

44

www.ck12.org

determine the wavelength by measuring the distance from one trough to the next or from the

top (crest) of one wave to the crest of the next wave. The symbol used for wavelength is the

Greek letter lambda, .

Another important characteristic of waves is called frequency. The frequency of a

wave is the number of waves that pass a given point each second. If we choose an exact

position along the path of the wave and count how many waves pass the position each

second, we would get a value for frequency. Frequency has the units of cycles/sec or

waves/sec, but scientists usually just use units of 1/sec or Hertz (Hz).

All types of light (EMR) travels at the same speed, 3.00ڄ108

m/s. Because of this, as

the wavelength increases (the waves get longer), the frequency decreases (fewer waves pass).

On the other hand, as the wavelength decreases (the waves get shorter), the frequency

increases (more waves pass).

Electromagnetic waves (light waves) have an extremely wide range of wavelengths,

frequencies, and energies. The electromagnetic spectrum is the range of all possible

frequencies of electromagnetic radiation. The highest energy form of electromagnetic waves

is gamma rays and the lowest energy form (that we have named) is radio waves.

On the far left of the figure above are the electromagnetic waves with the highest

energy. These waves are called gamma rays and can be quite dangerous in large numbers to

living systems. The next lowest energy form of electromagnetic waves is called x-rays. Most

of you are familiar with the penetration abilities of these waves. They can also be dangerous

to living systems. Next lower, in energy, are ultraviolet rays. These rays are part of sunshine

and rays on the upper end of the ultraviolet range can cause sunburns and eventually skin

cancer. The tiny section next in the spectrum is the visible range of light. These are the

frequencies (energies) of the electromagnetic spectrum to which the human eye responds.

The highest form of visible light energy is violet light, with red light having the lowest

energy of all visible light. Even lower in the spectrum, too low in energy to see, are infrared

rays and radio waves.

CC – Tracy Poulsen

45

www.ck12.org

The light energies that are in the visible range are electromagnetic waves that cause

the human eye to respond when those frequencies enter the eye. The eye sends signals to the

brain and the individual "sees" various colors. The highest energy waves in the visible region

cause the brain to see violet and as the energy of the waves decreases, the colors change to

blue, green, then to yellow, orange, and red. When the energy of the wave is above or below

the visible range, the eye does not respond to them. When the eye receives several different

frequencies at the same time, the colors are blended by the brain. If all frequencies of light

strike the eye together the brain sees white, and if there are no frequencies striking the eye

the brain sees black.

All the objects that you see around you are light absorbers – that is, the chemicals on

the surface of the objects absorb certain frequencies and not others. Your eyes detect the

frequencies that strike your eye. Therefore, if your friend is wearing a red shirt, it means that

the dye in that shirt absorbs every frequency except red and the red is reflected. When the red

frequency from the shirt strikes your eye, your visual system sees red and you say the shirt is

red. If your only light source was one exact frequency of blue light and you shined it on a

shirt that absorbed every frequency of light except one exact frequency of red, then the shirt

would look black to you because no light would be reflected to your eye. The light from

many fluorescent types of light do not contain all the frequencies of sunlight and so clothes

inside a store may appear to be a slightly different color than when you get them home.

Lesson Summary

Wave form energy is characterized by velocity, wavelength, and frequency.

As the wavelength of a wave increases, its frequency decreases. Longer waves with

lower frequencies have lower energy. Shorter waves with higher frequencies have

higher energy.

Electromagnetic radiation has a wide spectrum, including low energy radio waves and

very high energy gamma rays.

The different colors of light differ in their frequencies (or wavelengths).

Vocabulary

Frequency of a wave: The number of waves passing a specific point each second.

Wavelength: The distance between a point on one wave to the same point on the next

wave (usually from crest to crest or trough to trough).

Electromagnetic spectrum: A list of all the possible types of light in order of

decreasing frequency, or increasing wavelength, or decreasing energy. The

electromagnetic spectrum includes gamma rays, X-rays, UV rays, visible light, IR

radiation, microwaves and radio waves.

2.5: Review Questions

1) Which color of visible light has the longer wavelength, red or blue?

2) What is the relationship between the energy of electromagnetic radiation and the

frequency of that radiation?

3) Of the two waves drawn below, which one has the most energy? How do you know?

46

www.ck12.org

4) List the following parts of the electromagnetic spectrum in order of INCREASING

energy: radio, gamma, UV, visible light, and infrared

5) List the visible colors of light in order of INCREASING energy.

2.6: Atoms and Electromagnetic Spectra

Objectives

Describe the appearance of an atomic emission spectrum.

Explain that each element has a unique emission spectrum.

Explain how an atomic (or emission) spectrum can be used to identify elements

Describe an electron cloud that contains Bohr's energy levels.

Explain the process through which an atomic spectrum is emitted according to Bohr's

model of atoms.

Introduction

Electric light bulbs contain a very

thin wire in them that emits light when

heated. The wire is called a filament. The

particular wire used in light bulbs is

made of tungsten. A wire made of any

metal would emit light under these

circumstances but tungsten was chosen

because the light it emits contains

virtually every frequency and therefore,

the light emitted by tungsten appears

white. A wire made of some other element would emit light of some color that was not

convenient for our uses. Every element emits light when energized by heating or passing

electric current through it. Elements in solid form begin to glow when they are heated

sufficiently and elements in gaseous form emit light when electricity passes through them.

This is the source of light emitted by neon signs and is also the source of light in a fire.

Each Element Has a Unique Spectrum

The light frequencies emitted by atoms are mixed together by our eyes so that we see

a blended color. Several physicists, including Angstrom in 1868 and Balmer in 1875, passed

the light from energized atoms through glass prisms in such a way that the light was spread

out so they could see the individual frequencies that made up the light.

The emission spectrum (or atomic spectrum) of a chemical element is the unique

pattern of light obtained when the element is subjected to heat or electricity.

When hydrogen gas is placed into a tube and electric current passed through it, the

color of emitted light is pink. But when the color is spread out, we see that the hydrogen

The light emitted by the sign containing neon gas (on the

left) is different from the light emitted by the sign

containing argon gas (on the right).

Atomic spectrum of hydrogen

47

www.ck12.org

spectrum is composed of four individual frequencies. The pink color of the tube is the result

of our eyes blending the four colors. Every atom has its own characteristic spectrum; no two

atomic spectra are alike. The image below shows the emission spectrum of iron. Because

each element has a unique emission spectrum, elements can be identified using them.

You may have heard or read about scientists discussing what elements are present in

the sun or some more distant star, and after hearing that, wondered how scientists could

know what elements were present in a place no one has ever been. Scientists determine what

elements are present in distant stars by analyzing the light that comes from stars and finding

the atomic spectrum of elements in that light. If the exact four lines that compose hydrogen's

atomic spectrum are present in the light emitted from the star, that element contains

hydrogen.

Bohr's Model of the Atom

By 1913, the evolution of our

concept of the atom had proceeded

from Dalton's indivisible spheres

idea to J. J. Thomson's plum

pudding model and then to

Rutherford's nuclear atom theory.

Rutherford, in addition to

carrying out the brilliant experiment

that demonstrated the presence of the

atomic nucleus, also proposed that

the electrons circled the nucleus in a

planetary type motion. The solar

system or planetary model of the

atom was attractive to scientists because it was similar to

something with which they were already familiar, namely the

solar system.

Unfortunately, there was a serious flaw in the

planetary model. It was already known that when a charged

particle (such as an electron) moves in a curved path, it gives

off some form of light and loses energy in doing so. This is,

after all, how we produce TV signals. If the electron circling

the nucleus in an atom loses energy, it would necessarily have

to move closer to the nucleus as it loses energy and would

eventually crash into the nucleus. Furthermore, Rutherford's

model was unable to describe how electrons give off light

forming each element's unique atomic spectrum. These

difficulties cast a shadow on the planetary model and

indicated that, eventually, it would have to replaced.

Over time, our understanding of the atom has evolved.

Dalton's model (on the left) was altered when Thomson

discovered the electron and proposed the plum pudding

model (in the middle). Rutherford discovered the nucleus

and altered the model to the one on the right. Since then,

Neils Bohr and other scientists discovered more about the

location and energy of the electrons.

Niels Bohr and Albert

Einstein in 1925. Bohr

received the Nobel prize for

physics in 1922.

Atomic spectrum of iron.

48

www.ck12.org

In 1913, the Danish physicist Niels Bohr proposed a model of the electron cloud of an

atom in which electrons orbit the nucleus and were able to produce atomic spectrum.

Understanding Bohr's model requires some knowledge of electromagnetic radiation (or

light).

Energy Levels

Bohr's key idea in his model of the atom

is that electrons occupy definite orbits that

require the electron to have a specific amount of

energy. In order for an electron to be in the

electron cloud of an atom, it must be in one of

the allowable orbits and it must have the precise

energy required for that orbit. Orbits closer to

the nucleus would require smaller amounts of

energy for an electron and orbits farther from

the nucleus would require the electrons to have a

greater amount of energy. The possible orbits

are known as energy levels. One of the

weaknesses of Bohr's model was that he could

not offer a reason why only certain energy levels

or orbits were allowed.

Bohr hypothesized that the only way electrons could gain or lose energy would be to

move from one energy level to another, thus gaining or losing precise amounts of energy.

The energy levels are quantized, meaning that only specific amounts are possible. It would

be like a ladder that had rungs only at certain heights. The only way you can be on that ladder

is to be on one of the rungs and the only way you could move up or down would be to move

to one of the other rungs. Suppose we had such a ladder with 10 rungs. Other rules for the

ladder are that only one person can be on a rung and in normal state, the ladder occupants

must be on the lowest rung available. If the ladder had five people on it, they would be on the

lowest five rungs. In this situation, no person could

move down because all the lower rungs are full.

Bohr worked out rules for the maximum number of

electrons that could be in each energy level in his

model and required that an atom is in its normal

state (ground state) had all electrons in the lowest

energy levels available. Under these circumstances,

no electron could lose energy because no electron

could move down to a lower energy level. In this

way, Bohr's model explained why electrons circling

the nucleus did not emit energy and spiral into the

nucleus.

Bohr's Model and Atomic Spectra

The evidence used to support Bohr's model

came from the atomic spectra. He suggested that an

atomic spectrum is made by the electrons in an

Bohr proposed that electrons have specific

locations (or energy levels) around the nucleus,

much like there