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chapter
14
3
Inside the Atom
2
section ●
The Nucleus
Before You Read
Every person is different. What are some things that make
one person look different from another person?
What You’ll Learn
■
what radioactive
decay is
■ what half-life means
■ how radioactive
isotopes are used
Read to Learn
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Identifying Numbers
The electron cloud model is an example of what an average
nuclear atom looks like. But what makes atoms of different
elements different? The atoms of different elements have
different numbers of protons. The atomic number of an
element is the number of protons in the nucleus of an atom
of that element. The element hydrogen has the smallest atomic
number. It has only one proton in its nucleus, so hydrogen’s
atomic number is 1. The element uranium has the greatest
atomic number of a naturally occurring element. It has 92
protons in its nucleus. Its atomic number is 92.
Highlight Main Ideas As
you read, highlight the main
ideas under each heading. After
you finish reading, review the
main ideas of the lesson.
How many neutrons are in the nucleus?
A certain type of atom can have different numbers of
neutrons in its nucleus. For example, most carbon atoms
have six protons and six neutrons. But, some have seven or
eight neutrons. All of these atoms are carbon atoms because
they all have six protons.
These carbon atoms with different numbers of neutrons
are called isotopes. Isotopes (I suh tohps) are atoms of the
same element that have different numbers of neutrons.
Carbon-12 is an isotope that has 6 protons and 6 neutrons.
Carbon-13 has 6 protons and 7 neutrons. Carbon-14 has
6 protons and 8 neutrons. Together, the protons and
neutrons make up most of the mass in an atom.
C Build Vocabulary
●
Make the following Foldable and
write the definitions of each of
the terms from this lesson.
fe
Half-Li
Reading Essentials
215
What is the mass number?
The mass number of an isotope is the number of neutrons
plus protons. The table below shows the mass number and
particles for the isotopes of carbon. For example, carbon-12
has six protons and six neutrons, so its mass number is 12.
Notice that all isotopes of carbon have six protons. The
atomic number of carbon is 6.
Applying Math
1.
Evaluate How many
more neutrons does
Carbon-14 have than
Carbon-12?
Isotopes of Carbon
n-14
Mass number
4
Number of proto
6
Number of neutr
8
Number of electr
6
Atomic number
6
How do you hold things together? You might use tape or
glue. What holds the protons and neutrons together in the
nucleus of an atom? Remember that protons have a positive
charge. You might think that the protons would repel each
other. But, when the protons and neutrons in a nucleus are
tightly packed together, an even stronger binding force takes
over. This force is called the strong nuclear force. The strong
nuclear force keeps the protons from repelling each other.
This force works only in the nucleus of an atom.
2.
Explain What keeps the
protons in an atom from
repelling each other?
216
Inside the Atom
Radioactive Decay
The nuclei of most elements are stable because they have
about the same number of protons as neutrons. For
example, carbon-12 is stable because its atoms have six
protons and six neutrons. Some nuclei are unstable because
they have too many or too few neutrons. So the particles in
the nucleus try to repel each other. The nucleus must eject,
or release, a particle to become stable. When a nucleus lets
a particle go, it gives off energy. Radioactive decay is the
release of particles and energy from the nucleus.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
What is the strong nuclear force?
Transmutation When protons are released from the nucleus,
the atomic number of the atom changes. So one element
changes into another. Transmutation is the changing of
one element into another through radioactive decay.
3.
Explain What happens
to an element if it
undergoes transmutation?
4.
Define What are alpha
What happens when alpha particles are lost?
Most smoke detectors contain the element americium-241
(a muh RIH shee um). This element is unstable and
undergoes radioactive decay. Americium-241 transmutates
into another element by ejecting an alpha particle and
energy. An alpha particle is a particle that is made up of
two protons and two neutrons. The energy and alpha
particle that are ejected are called nuclear radiation.
In a smoke detector, the alpha particles make it possible
for the air to conduct an electric current. As long as the electric current flows, the detector is silent. Smoke will interrupt
the flow of the electric current and the alarm will go off.
particles made of?
How does an element change its identity?
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Americium has 95 protons. After transmutation, it only has
93 protons and becomes the element neptunium. Neptunium
has an atomic number of 93. Notice in the figure that the
mass and atomic numbers of neptunium and the alpha
particle add up to the mass and atomic number of americium.
No particles were destroyed during transmutation.
93 p
144 n
95 p
146 n
2 p
2 n
p n
p
n
Alpha
particle
Americium
Applying Math
5.
Calculate The element
actinium has an atomic
number of 89. How many
alpha particles would
americium need to lose
before it became actinium?
Neptunium
What are beta particles?
Not all transmutations cause the nucleus to eject an alpha
particle. Some eject an electron called a beta particle. A
beta particle is a high-energy electron that comes from the
nucleus, not the electron cloud. But, the nucleus contains
only protons and neutrons. How can it give off an electron?
In this kind of transmutation, a neutron becomes unstable.
It splits into an electron and a proton. The electron, or beta
particle, is ejected with a large amount of energy.
Reading Essentials
217
What happens to the proton?
After the electron is ejected, the proton stays in the
nucleus. Now there is one more proton in the nucleus and
the atomic number increases by one. The figure shows
unstable hydrogen-3. One neutron splits into a proton and
an electron. The electron (e–), or beta particle, is ejected.
Now the nucleus has two protons. Hydrogen-3 turns into
helium-3. The mass of the atom stays almost the same
because the mass of the electron it loses is so small.
Picture This
6.
Identify Circle the beta
1
2
2
1
eⴚ
particle in this diagram.
Hydrogen-3
Helium-3 ⴙ eⴚ
Applying Math
7.
Calculate If you start
with 4 g of iodine-131, how
many grams would there be
after 24 days? Show your
work.
Have you ever watched popcorn pop? You never know
which kernel will pop next. But, if you have popped a lot of
popcorn before, you might be able to predict how long it
will take for half the kernels to pop. Radioactive decay also is
random. That’s why radioactive decay is measured using its
half-life. The half-life of a radioactive element is the amount
of time it takes for half of a sample of the element to decay.
How do you calculate half-life decay?
The half-lives of radioactive isotopes range from fractions
of a second to billions of years. Iodine-131 has a half-life of
eight days. If you start with 4 g of iodine-131, after eight
days you have only half the amount, or 2 g. After eight more
days, you have only 1 g. The radioactive decay of unstable
atoms happens at a steady rate that nothing can change.
How are objects dated using half-life?
Scientists use radioactive decay to find the age of fossils.
Carbon-14 is a radioactive isotope of carbon. Its half-life is
5,730 years. It is used to find the age of dead animals, plants,
and humans. Living things have carbon-14 in them because
they take in and release carbon. When a living thing dies, the
amount of carbon-14 inside it begins to decrease because of
radioactive decay. Scientists can measure the amount of
carbon-14 in an ancient item. Using the half-life of carbon-14,
scientists can calculate when the animal, plant, or human lived.
218
Inside the Atom
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Rate of Decay
Why are long half-lives sometimes a problem?
Some radioactive isotopes have half-lives that are thousands,
millions, or billions of years. Waste products that have these
isotopes can be dangerous because they still release radiation.
These waste products must be kept away from people and the
environment. Special disposal sites are used to store this waste
for long periods. Many of these sites are deep underground.
Making Synthetic Elements
There are only 92 elements found in nature. Other
elements are made through transmutation. Scientists can
smash alpha and beta particles into the nuclei of existing
atoms to make new elements. Since these new elements are
made by humans, they are called synthetic. Synthetic
elements have greater numbers of protons and neutrons.
They have atomic numbers greater than 92.
8.
Define What are
synthetic elements?
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
What are the uses of radioactive isotopes?
Scientists have made many useful isotopes. These isotopes,
called tracer elements, can be placed in the body or released
into the air. Then, scientists can use instruments to look for
radiation while the tracer elements decay. Tracer elements
have been used to diagnose diseases and study the
environment. Tracer elements with short half-lives are the
best to use. The short half-lives do not expose living
organisms to radiation for long periods of time.
Diagnose Diseases Iodine is an element that is used by the
thyroid gland. Radioactive iodine-131 can be given to a
patient with a thyroid problem. The tracer element is
absorbed by the thyroid gland. The radiation can create a
picture of the thyroid. Doctors can then find out if the
patient’s thyroid is working properly.
Study the Environment Tracer elements are used in the
environment, too. Scientists inject them into the roots of
plants to see how the roots absorb food. Others are put into
pesticides. The tracer elements can then be followed to find
out what happens to the pesticide in the environment.
9.
Infer Why might we want
to know what happens to
pesticides in the
environment?
Reading Essentials
219
After You Read
Mini Glossary
atomic number: the number of protons in the nucleus of an
atom of an element
beta particle: a high-energy electron that comes from the
nucleus, not the electron cloud
half-life: the amount of time it takes for half of a sample of
the element to decay
isotopes: atoms of the same element that have different
numbers of neutrons
mass number: the number of neutrons plus protons
radioactive decay: the release of particles and energy from
the nucleus
transmutation: the changing of one element into another
through radioactive decay
1. Review the terms and their definitions in the Mini Glossary. Explain why the mass number and atomic number of an element are different. Use complete sentences.
Radioactive Decay
Type of Particle Released
Decay Process
Result
Alpha particle
Beta particle
End of
Section
220
Inside the Atom
The atom changes into a different
element. The atom now has a lower
atomic number.
A neutron in the nucleus splits
into a proton and an electron. The
electron, or beta particle, is ejected.
Visit blue.msscience.com to access your textbook, interactive
games, and projects to help you learn more about the nucleus
of an atom.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
2. Complete the table to explain what causes radioactive decay and what happens when
particles are ejected.