Name
Class
CHAPTER 4
Date
Atomic Energy
1 Radioactivity
SECTION
National Science
Education Standards
BEFORE YOU READ
After you read this section, you should be able to answer
these questions:
PS 3a, 3e
• What are three types of radioactive decay?
• How does radiation affect living and non-living things?
• How do people use radioactive materials?
How Was Radiation Discovered?
In 1896, a French scientist named Henri Becquerel performed an experiment to test a hypothesis. He thought that
certain materials made X rays when light shone on them.
To test his idea, he wrapped a photographic plate in
black paper to protect it from sunlight. Then he placed a
mineral that glows when it is exposed to light on top of
the paper. Becquerel placed the photographic plate and
the mineral in the sun. When he developed the plate, he
saw an image of the mineral as shown below.
STUDY TIP
Compare In a table, list the
types of radioactive decay, the
particle given off, how penetrating it is, and how it is useful.
Sunlight could not pass through
the paper. The image on the plate
must have come from energy
given off by the mineral.
When Becquerel tried to do the experiment again,
the weather was cloudy, so he put his materials away in
a drawer. He decided to develop the plate anyway and
found a surprising result. Even without sunlight, an image
of the mineral formed on the photographic plate. He
repeated the test with the same result. He concluded that
some kind of energy came from uranium, an element in
the mineral.
The energy was nuclear radiation, high-energy particles that come from the nuclei of some atoms. A scientist
who worked with Becquerel, Marie Curie, named the process that causes nuclear radiation. That process is called
radioactivity. It is also known as radioactive decay.
READING CHECK
1. Identify What caused
the image of the mineral in
Becquerel’s experiment to form
on the photographic plate?
2. Identify What is another
name for radioactivity?
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Atomic Energy
Name
SECTION 1
Class
Date
Radioactivity continued
What Are the Types of Radioactivity?
The nuclei of some atoms are not stable. During radioactive decay, an unstable nucleus changes and gives off
particles and energy. There are three types of radioactive
decay, called alpha, beta, and gamma.
ALPHA DECAY
READING CHECK
3. Describe What is an
alpha particle made of? What
is mass number?
An alpha particle is made of two protons and two neutrons. The mass number of a nuclear particle or a nucleus is
the sum of the numbers of protons and neutrons. The mass
number of an alpha particle is 4 and its charge is 2. An
alpha particle is also labeled helium-4, because a helium-4
nucleus has two protons and two neutrons.
The release of an alpha particle from a nucleus is
called alpha decay. When an alpha particle is released
from a nucleus, it changes into the nucleus of another
element. See the figure below.
Alpha Decay of Radium-226
Radon-222
Radium-226
Mass number is
conserved.
226  222  4
Energy
Charge: 86
Math Focus
4. Determine If the mass
number of radium were
228, what would be the
mass number of the radon
formed?
READING CHECK
5. Identify What is
conserved by the radioactive
decay process?
Charge is conserved.
(88)  (86)  (2)
Alpha particle
(helium-4)
Charge: 88
Charge: 2
Many large radioactive nuclei break apart by releasing
an alpha particle. When a nucleus emits an alpha particle,
it becomes the nucleus of a different element because the
new nucleus has a different number of protons. Radium-226
becomes radon-222.
The model of alpha decay in the figure shows two
important facts about radioactive decay. The first is that
mass is conserved. Radium-226 has the same mass as
radon-222 plus helium-4.
The second fact is that charge is conserved. A radium
nucleus has a charge of 88. If you add the charge of
radon, 86, and an alpha particle, 2, you get 88.
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Atomic Energy
Name
Class
SECTION 1
Date
Radioactivity continued
BETA DECAY
A beta particle is an electron or positron that is
released from an atomic nucleus. An electron has a
charge of 1. A positron has a charge of 1. Both particles have a mass that is very close to zero compared to
the mass of a nucleus. That means the mass of a nucleus
does not change when it emits a beta particle.
The beta decay shown in the figure below is the loss of
an electron from a carbon-14 nucleus. During this kind of
decay, one of the neutrons becomes a proton and an electron leaves the nucleus. Mass and charge are conserved.
The number of protons changes so the nucleus becomes
a different element.
READING CHECK
6. Describe What happens
to the mass of the nucleus
that emits a beta particle?
Beta Decay of Carbon-14
Carbon-14
Nitrogen-14
Mass number is conserved.
14  14  0
Energy
Charge is conserved.
(6)  (7)  (1)
Charge: 7
Critical Thinking
7. Infer What would have
been the charge of the new
nucleus formed if the beta
particle had been a positron?
Beta particle
(electron)
Charge: 6
Charge: 1
Isotopes are atoms that have the same number of protons but different number of neutrons. Different isotopes of
an element can decay in different ways. A carbon-11 nucleus
decays by emitting a positron. Again, the overall mass and
charge do not change. The decay of carbon-11 creates a
nucleus of boron-11, which has 5 protons and 6 neutrons.
READING CHECK
8. Describe What are
isotopes?
GAMMA DECAY
Some changes to a nucleus also emit energy in the
form of high-energy waves. These waves are called
gamma rays. During gamma decay, particles in the
nucleus move and change position, but there is no change
of mass or charge. That means one element does not
change into another. Gamma decay often occurs at the
same time as alpha or beta decay.
READING CHECK
9. Describe What are
gamma rays?
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Atomic Energy
Name
SECTION 1
Class
Date
Radioactivity continued
How Does Radiation Affect Matter?
READING CHECK
10. Explain Why are alpha
particles less penetrating
than beta particles?
Because the particles and rays of nuclear radiation
have a lot of energy, they can move through matter. As
shown in the figure below, each type of radiation has a
different ability to penetrate, or go through, matter. This
penetration depends on charge and on mass.
Alpha particles have the most mass and charge so they
tend to interact with atoms more easily. Alpha particles
are stopped by a piece of paper or clothing. A beta particle has almost no mass and a single charge. Clothing does
not stop beta particles but about 3 mm of a metal such as
aluminum can.
Gamma rays have no charge or mass. They pass
through metals such as aluminum. Only very dense, thick
materials can stop gamma rays. They can be blocked by a
few centimeters of lead or a few meters of concrete.
The Penetrating Abilities of Nuclear Radiation
Radioactive
material
Alpha
particles
Gamma
rays
Beta
particles
STANDARDS CHECK
PS 3e In most chemical and
nuclear reactions, energy is
transferred into or out of a
system. Heat, light, mechanical
motion, or electricity might all
be involved in such transfers.
11. Describe How can
gamma rays cause changes
deep inside matter?
Say It
Describe Research the
effects of alpha, beta, and
gamma radiation on a living
cell. Describe to the class
what each type can do to a cell.
Paper
Alpha particles have a greater
charge and mass than beta
particles and gamma rays do.
Alpha particles travel about
7 cm through air and are
stopped by paper or clothing.
Aluminum
Beta particles have a 1 or
1 charge and almost no
mass. They are more penetrating than alpha particles. Beta
particles travel about 1 m
through air but are stopped
by 3 mm of aluminum.
Concrete
Gamma rays have no charge
or mass and are the most
penetrating. They are blocked
by very dense, thick materials,
such as a few centimeters of
lead or a few meters of
concrete.
When nuclear radiation hits atoms, atoms can lose
electrons. Radiation can also break bonds between
atoms. These changes can cause damage to matter. The
amount and location of the changes depends on the type
of radiation.
Gamma rays can cause changes deep inside matter because they penetrate deeply. Beta radiation does
not penetrate as far, so it causes damage closer to the
surface. Alpha particles are much larger and have more
charge than beta particles. Although they are easier to
stop, alpha particles cause the most damage when they
get inside matter.
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Atomic Energy
Name
SECTION 1
Class
Date
Radioactivity continued
DAMAGE TO LIVING MATTER
Radiation can harm the cells of a living organism.
The damage is often similar to a burn caused by touching something hot. A large exposure to radiation causes
radiation sickness. The symptoms of this sickness include
fatigue, loss of appetite, and hair loss. When blood cells
are destroyed, radiation sickness can cause death.
Repeated exposure to radiation can damage many cells
in the body. One of the effects of this damage can be cancer. Many people who work near radiation wear badges
that can warn them when radiation levels are too high.
DAMAGE TO NONLIVING MATTER
Radiation can also damage nonliving matter. For
example, when electrons are knocked away from metal
atoms, the metal becomes weaker. The metal structures
in nuclear power plants must be tested often. Too much
exposure to radiation can make them unsafe. Parts in
spacecraft can be changed by radiation from the sun.
READING CHECK
12. Explain Why do people
exposed to radiation regularly
need to wear radiation badges?
How Do People Use Radioactivity?
Radiation can be harmful but it can also be useful in
industry, in medicine, and even in your home. Some smoke
detectors use a tiny amount of radioactive material. It ionizes
atoms in smoke and the ions turn on the alarm.
Radioactive materials can also be used as tracers.
Tracers are radioactive elements whose paths can be followed through a process such as a chemical reaction.
READING CHECK
13. Describe What do
tracers allow people to do?
RADIOACTIVITY IN HEALTH CARE
Doctors use tracers to help find patient’s medical
problems. The figure below shows an image of a thyroid
gland. The image shows parts of the gland are not working correctly. Radioactive materials are also used to treat
diseases, including cancer.
Radioactive iodine -131 was used
to make this scan of a thyroid
gland. The dark area shows the
location of a tumor.
TAKE A LOOK
14. Identify Circle the dark
area in the thyroid shown in
the figure.
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Atomic Energy
Name
SECTION 1
Class
Date
Radioactivity continued
RADIOACTIVITY IN INDUSTRY
READING CHECK
Radioactive isotopes are also used as tracers in industry.
The figure below shows a worker looking for leaks in pipes
by tracing radioactive material in the pipe. Notice that he is
wearing protective clothing so his cells are not exposed to
radiation. Another way to use radiation is to look for flaws
in metal objects. This is similar to the way a doctor uses an
X-ray image to look at your bones.
15. Identify What are two
uses for tracers in industry?
Tracers are used to find
weak spots in materials
and leaks in pipes. A
Geiger counter is often
used to detect the tracer.
Some space probes use radioactive isotopes for power.
The energy given off as nuclei decay is converted to electrical power.
How Can Radiation Tell Us About the Past?
In 1991, hikers in the Alps found a frozen body high in
mountains. Scientists used radioactivity to figure out that
the Iceman, shown below, lived about 5,300 years ago.
The Iceman is a
5,300-year-old mummy.
His are the best-preserved
remains of a human from
that time.
READING CHECK
16. Identify What radioactive
isotope of carbon is not replaced after an organism dies?
Every living thing has many carbon atoms. A small
percentage of these atoms is radioactive carbon-14. This
percentage does not change in a living organism because
carbon atoms are constantly being replaced. When the
organism dies, though, it no longer replaces atoms. As the
radioactive isotope decays, the percentage of carbon-14
decreases. Scientists can figure out when the organism
was alive by measuring how much carbon-14 remains.
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Atomic Energy
Name
SECTION 1
Class
Date
Radioactivity continued
Radioactive Decay and Half-Life
The original sample contains a certain amount of
radioactive isotope.
,
After
one-half of the original
sample has decayed, and
half is unchanged.
,
After
one-fourth of the original
sample is still unchanged.
Math Focus
17. Identify Fill in the blanks
to identify which half-life
(one, two, three, etc.) the
sample has decayed through.
A STEADY DECAY
Every radioactive isotope decays at a particular rate,
called its half-life. As shown in the figure above, a half-life
is the amount of time that it takes for one-half the nuclei of
a radioactive isotope to decay.
The half-life is the same for every sample of a particular isotope. It is not affected by any conditions such as
temperature and pressure. As shown in the table, halflives range from part of a second to billions of years.
Examples of Half-lives
Isotope
Half-life
Isotope
Half-life
Uranium-238
4.5 billion years
Polonium-210
138 days
Oxygen-21
3.4s
Nitrogen-13
10 min
Hydrogen-3
12.3 years
Calcium-36
0.1s
FINDING AGE
The half-life of carbon-14 is 5,730 years. This rate is constant. Scientists know what percentage of the carbon in the
Iceman’s body was carbon-14 when he was alive. They measured the number of decays per minute in a sample from his
body. This showed what the percentage of carbon-14 is now.
A little less than half of the carbon-14 had decayed
after his death. That means that not quite one half-life has
passed since the Iceman walked in the mountains.
Carbon-14 can be used to find the age of objects up to
50,000 years old. After that, there is not enough carbon14 left to make good measurements. To find the age of
older things, scientists use isotopes with longer half-lives.
For example, the half-life of potassium-40 is about 1.3 billion years. It is used to find the age of dinosaur fossils.
Isotopes with very long half-lives, such as uranium-238,
are used to measure the age of Earth’s oldest rocks.
READING CHECK
18. Describe Why do scientists think that the Iceman is
less than 5,730 years old?
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Atomic Energy
Name
Class
Date
Section 1 Review
NSES
PS 3a, 3e
SECTION VOCABULARY
half-life the time required for half of a sample
of a radioactive isotope to break down by
radioactive decay to form a daughter isotope
isotope an atom that has the same number of
protons (or the same atomic number) as other
atoms of the same element but has a different number of neutrons (and thus a different
atomic mass)
mass number the sum of the numbers of
protons and neutrons in the nucleus of an
atom.
radioactivity the process by which an unstable
nucleus gives off nuclear radiation
1. Compare Most atoms of uranium, which has 92 protons, are either uranium-235 or
uranium-238. Compare the mass numbers and atomic numbers of these two isotopes of
uranium. Recall, atomic number is the number of protons in the nuclei of elements.
2. Compare Use the information in this section to fill in the blank spaces in the
comparison table below.
Types of Radiation
Name
Form
Mass
particle (helium
nucleus)
Beta
Charge
2+
0
particle
(positron)
Penetrating
power
medium
0
1+
medium
energy
3. Make Inferences Nuclear radiation can be used to look for flaws in metal parts of
bridges. The process is similar to using X rays to look at human bones. What type
of radioactive decay would work best for this test? Explain your answer.
4. Evaluate Results A rock contains one-fourth of its original potassium-40. The half-
life of potassium 40 is 1.3 billion years. What is the rock’s age?
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Atomic Energy
L
Interactions of Matter Answer Key continued
12. the information that the cell needs to build
14. Dark area should be circled.
15. Check for leaks in pipes and flaws in metal
protein molecules
Review
1. Type of carbon
backbone
objects.
16. carbon-14
17. one half-life, two half-lives
18. A little less than half of the carbon-14 had
decayed after his death.
Description
Ring
The chain of carbon atoms forms
a ring.
Straight chain
All carbon atoms are connected in
a straight line.
Branched chain
The chain of carbon atoms
separates into different
directions.
Review
1. The atomic number for both isotopes is 92.
The mass number of uranium-235 is 235, and
the mass number of uranium-238 is 238.
2. First column, top to bottom: alpha, beta,
gamma
Second column: particle (electron)
Third column, top to bottom: 4, 0
Fourth column, top to bottom: 1-, 0
Last column, top to bottom: low, high
3. Gamma rays would work because they have
the most penetrating power and enough
energy to pass through metal. Alpha and
beta particles would be stopped by the
metal parts.
4. One-fourth remaining indicates 2 half-lives
or 2.6 billion years.
2. saturated compounds—alkanes,
unsaturated compounds—alkenes and alkynes
3.
Type of biochemical
Description
Proteins
made of hundreds or
thousands of amino acid
molecules
Nucleic acids
one of the functions is to
store genetic information
Carbohydrates
made of one or more
simple sugar molecules
Lipids
one of the functions is to
store energy
4. DNA—contains the genetic material of a cell;
RNA—contains the information that the cell
needs to build protein molecules.
SECTION 2 ENERGY FROM THE NUCLEUS
1. A large nucleus splits into two smaller
Chapter 4 Atomic Energy
nuclei, releasing energy.
2. Their difference is three because three neu-
SECTION 1 RADIOACTIVITY
1. Energy came from the uranium.
2. radioactive decay
3. An alpha particle has two protons and two
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
3.
4.
neutrons. Mass number is the sum of the
protons and neutrons in a nuclear particle or
in a nucleus.
224
mass and charge
It stays the same.
5+
atoms that have the same number of protons
but different number of neutrons
high-energy waves
They have more mass and charge, so they
tend to interact with atoms more easily.
They penetrate matter deeply.
to warn them if they have been exposed to
radiation harmful enough to damage cells
follow the path of a process
5.
6.
7.
8.
9.
10.
11.
12.
trons are also produced.
Some of the masses are changed into energy.
a continuous series of nuclear fission
reactions
by keeping some of the neutrons from hitting a uranium nucleus
kinetic energy changed into mechanical
energy, mechanical energy changed into
electrical energy
An explosion can blow a large amount
of radioactive fuel and waste into the
atmosphere.
It has dangerous levels of radioactivity.
carbon dioxide
Two or more nuclei that have small masses
combine to form a larger nucleus.
the core
Scientists cannot yet control the high temperatures well enough to use fusion.
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Interactions of Matter