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number is unchanged because the mass of the expelled
EXAMPLE 13.3 electron (beta particle) is negligible.
Would you predict the following isotopes to be radioactive or stable? Beta particles are more penetrating than alpha particles
60 and may travel several hundred centimeters through the
(a) Co
27

222 air. They can be stopped by a thin layer of metal close to the
(b) Rn
86
3 emitting nucleus, such as a 1 cm thick piece of aluminum.
(c) H
1
40 A spent beta particle may eventually join an ion to become
(d) Ca
20
part of an atom, or it may remain a free electron.
3. Gamma emission. Gamma (γ) emission is a high-energy
SOLUTION burst of electromagnetic radiation from an excited nucleus.
It is a burst of light (photon) of a wavelength much too
(a) Cobalt-60 has 27 protons and 33 neutrons, both odd numbers, so
60 short to be detected by the eye. Other types of radioactive
you might expect Co to be radioactive.

27
(b) Radon has an atomic number of 86, and all isotopes of all decay, such as alpha or beta emission, sometimes leave
elements beyond atomic number 83 are radioactive. Radon-222 is the nucleus with an excess of energy, a condition called an
therefore radioactive. excited state. As in the case of excited electrons, the nucleus
(c) Hydrogen-3 has an odd number of protons and an even num- returns to a lower energy state by emitting electromagnetic
ber of neutrons, but its 2:1 neutron-to-proton ratio places it radiation. From a nucleus, this radiation is in the high-
outside the band of stability. Hydrogen-3 is radioactive. energy portion of the electromagnetic spectrum. Gamma is
(d) Calcium-40 has an even number of protons and an even number
the most penetrating of the three common types of nuclear
of neutrons, containing 20 of each. The number 20 is a par-
radiation. Like X rays, gamma rays can pass completely
ticularly stable number of protons or neutrons, and calcium-40
through a person, but all gamma radiation can be stopped
has 20 of each. In addition, the neutron-to-proton ratio is 1:1,
by a 5 cm thick piece of lead close to the source. As with
placing it within the band of stability. All indications are that
other types of electromagnetic radiation, gamma radiation
calcium-40 is stable, not radioactive.
is absorbed by and gives its energy to materials. Since the
product nucleus changed from an excited state to a lower
TYPES OF RADIOACTIVE DECAY energy state, there is no change in the number of nucleons.
For example, radon-222 is an isotope that emits gamma
Through the process of radioactive decay, an unstable nucleus
radiation:
becomes a more stable one with less energy. The three more
222
0
222
familiar types of radiation emitted—alpha, beta, and gamma— Rn* → Rn + γ

86
0
86
were introduced earlier. There are five common types of radioactive
(*denotes excited state)
decay, and three of these involve alpha, beta, and gamma radiation.
Radioactive decay by alpha, beta, and gamma emission is
1. Alpha emission. Alpha (α) emission is the expulsion
4 summarized in Table 13.2, which also lists the unstable nuclear
of an alpha particle ( He) from an unstable, disintegrating conditions that lead to the particular type of emission. Just as

2
nucleus. The alpha particle, a helium nucleus, travels from

electrons seek a state of greater stability, a nucleus undergoes
2 to 12 cm through the air, depending on the energy of
radioactive decay to achieve a balance between nuclear attrac-
emission from the source. An alpha particle is easily stopped
tions, electromagnetic repulsions, and a low quantum of nuclear
by a sheet of paper close to the nucleus. As an example of
shell energy. The key to understanding the types of reactions
alpha emission, consider the decay of a radon-222 nucleus:
that occur is found in the band of stable nuclei illustrated in
222 218 4
Rn → Po + He Figure 13.5. The isotopes within this band have achieved the
2
86
84

The spent alpha particle eventually acquires two electrons
and becomes an ordinary helium atom. TABLE 13.2
–
2. Beta emission. Beta (β ) emission is the expulsion of

a different particle, a beta particle, from an unstable Radioactive decay
disintegrating nucleus. A beta particle is simply an Unstable Product
0

electron ( e) ejected from the nucleus at a high speed. Th e Condition Type of Decay Emitted Nucleus

–1
emission of a beta particle increases the number of protons 4
More than Alpha emission H Lost 2 protons
in a nucleus. It is as if a neutron changed to a proton by 2
83 protons and 2 neutrons
emitting an electron, or 0
Neutron-to- Beta emission e Gained 1 proton,
–1
1 1 0 proton ratio no mass change
n → p + e

–1
1
0
too large
Carbon-14 is a carbon isotope that decays by beta emission: Excited nucleus Gamma emission γ No change
0
0
14 14 0 Neutron-to- Other emission 0 e Lost 1 proton,
C → N + e 1
7
–1
6
proton ratio no mass change
Note that the number of protons increased from six to too small
seven, but the mass number remained the same. Th e mass
328 CHAPTER 13 Nuclear Reactions 13-6