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Date: ___________
Radioactive Decay
Unit: Nuclear Chemistry
Mass. Frameworks Section(s): 2.5 & 2.7
Knowledge/Understanding:

types of nuclear decay

types of particles & emissions created by nuclear decay

fission and fusion
Skills:

determining the products of radioactive decay
nuclear instability: If the nucleus of an atom is unstable, a piece of it splits
off. When the piece splits off, the nucleus ends up with a different
number of protons, which means the atom literally turns into an atom of a
different element.
Nuclei can become unstable if:
 they are too large (more than 82 protons)
 there are too many or too few neutrons for the number of protons in
the nucleus
radioactive decay: the process by which a nucleus loses matter, which
transforms the element into a different element or isotope.
nuclear equation: an equation describing (through chemical symbols) what
happens to an atom as it undergoes radioactive decay.
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Chemistry I
Copyright © 2009–2017 Mr. Bigler.
Permission is granted to copy, distribute and/or modify this document under the terms
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Radioactive Decay
band of stability: isotopes with a ratio of protons to neutrons that results
in a stable nucleus (one that does not spontaneously undergo radioactive
decay).
alpha (α) decay: a type of radioactive decay in which the nucleus loses two
protons and two neutrons (an alpha particle). An alpha particle is
identical to the nucleus of a 24He atom. For example:
4
U234
90Th  2 He
238
92
Atoms are likely to undergo alpha decay if they have an otherwise stable
proton/neutron ratio but a large atomic number (>82).
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Chemistry I
Mr. Bigler
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Radioactive Decay
beta minus (β−) decay: a type of radioactive decay in which the nucleus loses
a high speed electron (chemical symbol 10e ). In this process, a quark
flips from “down” to “up,” which changes a neutron into a proton. Because
a proton was gained, the atomic number increases by one. However,
because the proton used to be a neutron, the mass number does not
change. For example:
32
15
0
P 32
16 S -1 e
Atoms are likely to undergo β− decay if they have too many neutrons and
not enough protons to achieve a stable neutron/proton ratio.
electron capture (sometimes called “K-capture”): when the nucleus
“captures” an electron from the innermost shell (the K-shell) and a
neutrino, and incorporates them into the nucleus. This process is exactly
the reverse of β− decay; the electron capture causes a quark to flip from
“up” to “down” and changes the proton to a neutron.
Mg  10e 23
11 Na
23
12
Atoms are likely to undergo electron capture if they have too many
protons and not enough neutrons to achieve a stable neutron/proton
ratio.
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Chemistry I
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Radioactive Decay
beta plus (β+) decay: a type of radioactive decay in which the nucleus loses a
high speed positron (chemical symbol 10e ). In this process, a quark flips
from “up” to “down,” which changes a proton into a neutron. Because a
proton was lost, the atomic number decreases by one. However, because
the neutron used to be a proton, the mass number does not change. For
example:
0
Mg 23
11 Na  1 e
23
12
Atoms are likely to undergo β+ decay if they have too many protons and
not enough neutrons to achieve a stable neutron/proton ratio.
Note that β+ decay and electron capture produce the same products.
Electron capture can sometimes (but not often) occur without β+ decay,
but β+ decay is always accompanied by electron capture.
gamma (γ) rays: most radioactive decay produces a lot of energy. Some of
that energy is emitted in the form of gamma rays. Gamma rays are a high
energy form of light that can do a lot of damage. Because gamma rays
are not particles, emission of gamma rays does not change the
composition of the nucleus.
penetrating power: the distance that radioactive particles can penetrate
into/through another substance is directly related to the velocity of the
emission (faster = more penetrating) and inversely related to both the
mass of the emission (heaver = less penetrating) and the density of the
substance being penetrated (denser, e.g., lead = less penetration).
gamma rays > beta particles > alpha particles
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Mr. Bigler
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Radioactive Decay
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fission: the intentional splitting of the nucleus of an atom, usually by
bombarding it with a high-speed neutron. For example:
87
146
1
n  235
92 U 35 Br 57 La  3 0 n
1
0
In the above equation, 238U is smashed by a neutron, splitting it into
146
La, and 3 more neutrons.
87
Br,
fusion: the joining together of the nuclei of two atoms, accomplished by
colliding them at high speeds. For example:
H 12H24 He
2
1
the above fusion reaction takes place on the sun and other stars.
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Chemistry I
Mr. Bigler
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