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what are isotopes?
es
of an element is.
Compare the three isotopes of hydrogen.
Explain what an isotope
Kw
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isotope (EYE-suh-tohp): atom of an element
with the same number of protons as the other
atoms but a different number of neutrons
Different Atomic Masses
The atomic number
of an element never changes. All atoms of the same
element have the same number of protons in their
nuclei. However, all atoms of the same element
may not have the same number of neutrons in their
nuclei. This means that atoms of the same element
can have different atomic masses. The difference in
atomic mass is caused by a different number of
neutrons in the nuclei of the atoms of an element.
[f.
srnrr' What causes atoms of the same
element to have different atomic masses?
lsotopes Atoms of the same element that have
different atomic masses are called isotopes.
Isotopes are atoms of an element that have the
same number of protons as the other atoms of the
element but a different number of neutrons in their
nuclei.
Although the number of neutrons in the atoms
an
element may be different, atoms are always
of
identified by the number of protons. For example,
there are several isotopes of the element coPPer.
One of the isotopes has 36 neutrons in the nuclei of
its atoms. Another isotope has 34 neutrons in the
nuclei of its atoms. Because both of these isotopes
have 29 protons in the nuclei of their atoms, they
are both atoms of the element copper.
p"
orrtrrrr: What is an isotope?
The atomic mass for each element
canbe found in the periodic table. The atomic mass
given for each element is actually an average of the
atomic masses of all the isotopes of that element.
This explains why an element's atomic mass is not
a whole number.
Atomic Mass
What does the atomic mass for each
element actually represent?
EXPLAIN:
lsotopes
of
Common Elements Hydrogen
has three isotopes. The three isotopes of hydrogen
are known as protium (PROHT-ee-uhm),
deuterium (doo-TlR-ee-uhm), and tritium (TRIHTee-uhm). These isotopes of hydrogen are also
called hydrogen-1 (H-1), hydrogen-2 (H-2), and
hydrogen-3 (H-3). The numbers L, 2, and 3
represent the mass numbers of each of the
isotopes. An atom of protium (H-1) has only one
proton and no neutrons in its nucleus. An atom of
deuterium (H-2) has one proton and one neutron.
An atom of tritium (H-3) has one proton and two
neutrons.
4
Figure 3-33
The three isotopes
of hydrogen
Protium (H-1)
atom
Deuterium (H-2)
atom
Tritium (H-3)
atom
A11 elements have two or more isotopes.
-:rbon-12 and carbon-14 arc two isotopes of the
=-ement carbon. These isotopes are also written as
--12 and C-14. Boron-l0 (B-10) and boron-11 (B-11)
.:e isotopes of the element boron. TWo important
-:otopes of uranium are uranium-235 (U-235) and
::anium-238 (U-238).
p
The atomic number of carbon is 6.
How many protons and neutrons are there in
an atom of carbon-12? Of carbon-14?
6. CALCULATE:
number of oxygen is 16. Its
atomic mass is 15.999. The atomic number of
oxygen is 8. Which of the following statements
about the isotopes of oxygen are true? \AIhy?
7. ANALYZE: The mass
ttsr, What are the three isotopes of hydrogen
a. A11 of the isotopes have eight neutrons.
b. All of the isotopes have eight neutrons or
called?
more.
dcHrcrirue coyCrrrs
_ ". "__r]
c.
1. \Alhat causes some atoms of the same element
to have different atomic masses?
l.
In the periodic table, where can the atomic
mass of an element be found?
J.
What identifies an atom of an element?
How many neutrons are there in an atom of
protium? Deuterium? Tritium?
I
a,
Some of the isotopes have fewer than eigl-rt
neutrons.
d. A1l of the isotopes have fewer than eight
neutrons.
INTERPRETINGVISUALS
, .
,.
Use Figure 3-33 to ansLler the
8.
ANALYZE:
following questions.
Which isotope contains two neutrons
in its nucleus?
THINKtN_G CR|T|CATTY
Why is the atomic mass of an element
not a whole number?
EXPLAIN:
9.
Which atom has the largest nucleus?
10. INFER: What is the atomic number of each
IDENTIFY
isotope?
11.
INFER:
Which isotope has the smallest atomic
mass?
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Integratiry Lifo Soi,ena,
ToPIC: cells
KILLING CANCER CELLS
The human body is made up of trillions of cells. Cells divide
and organize themselves to keep the body healthy. They
can also replace themselves to promote repair. Cancer
occurs when cells cannot control their growth and
development. Cancer cells divide, multiply, and spread.
These cells can also invade organs. As they invade organs,
they damage healthy cells and send out substances that
weaken the body.
Research scientists have uncovered different ways to treat different types of cancer.
Radiation therapy is one method. One type of radiation therapy uses radioactive
isotopes to kill cancer cells. This type of treatment places radioactive isotopes close
to the cancer cells, The high-energy rays from the isotopes attack the cancer cells.
Some radioactive isotopes used in this type of therapy are radium -223, gallium-67,
iodine-l 23, and fluorine-18.
Thinking Critically How are radioactive isotopes
used
to treat certain types of .un."r?
",=
CHAPTER 3: Elements and
Atoms
73