Modes of Radioactive Decay
An unstable nucleus is radioactive, which means that it emits particles or
electromagnetic radiation in order to become more stable. The stability of a
nucleus depends on its size and on the ratio of neutrons to protons. A nucleus
is radioactive when it is too large or when the ratio of neutrons to protons is
either too large or too small.
Alpha () decay
Alpha decay occurs when a nucleus is too big. In order to get smaller, the
4
4
nucleus spits out an alpha particle which is a helium-4 nucleus ( 2 He or 2 α ).
A
Z
X→
Y+ α
A-4
4
Z- 2
2
Notice that alpha decay produces a nucleus with two less protons and 4 less
nucleons. All nuclei with an atomic number bigger than 83 are radioactive and
decay by alpha decay.
Beta () decay
Beta decay occurs when a nucleus has too many neutrons and too few protons
(the neutron to proton ratio is too high). The nucleus becomes more stable by
changing a neutron into a proton and an electron ( particle). The proton
remains in the nucleus and the beta particle is ejected at high speed.
A
Z
X→
A
Y+ 
Z1
0
-1
Notice that beta decay produces a nucleus with one more proton but with the
same number of nucleons.
Positron emission
Positron emission occurs when a nucleus has too few neutrons and too many
protons (the neutron to proton ratio is too low). The nucleus becomes more
stable by changing a proton into a neutron and a positron (positively charged
electron). The neutron remains in the nucleus and the positron is ejected at
high speed.
A
Z
X→ Y+ e
A
0
Z -1
1
Notice that positron emission produces a nucleus with one less proton but with
the same number of nucleons. As you might expect, positron emission and beta
decay have opposite effects!
Electron capture
There is a second way to convert a proton into a neutron. The nucleus can
capture an electron from the innermost orbital.
A
Z
X + e→ Y
0
A
-1
Z -1
Notice that the consequences of electron capture and positron emission are
identical. A nucleus with one less proton and the same number of nucleons is
produced. When a nucleus has a low neutron to proton ratio, either electron
capture or positron emission can occur. Larger nuclei are more likely to
undergo electron capture than are smaller nuclei. Do you see why?
Gamma () radiation
Gamma radiation can be associated with all of the four modes of decay
described above. When the new nucleus forms, its protons and neutrons might
not be in their most stable arrangement. Such a nucleus is said to be “metastable”. When the protons and neutrons in a meta-stable nucleus rearrange into
a more stable configuration, the excess energy is emitted in the form of a
photon of gamma radiation.
Am
Z
X → X +  (“m” stands for “meta-stable”)
A
Z