Nuclear Stability and Radioactive Decay
With a discussion of radioisotopes comes the topic of nuclear stability.
The nucleus of a radioisotope is unstable. In an attempt to reach a more
stable arrangement of its protons and neutrons, the nucleus will
spontaneously decompose to form a different nucleus. If the number of
neutrons changes in the process, a different isotopes is formed. If the
number of protons changes in the process, then an atom of a different
element is formed. This decomposition of the nucleus is referred to as
radioactive decay. During radioactive decay an unstable nucleus
spontaneosly decomposes to form a different nucleus, giving off radiation
in the form of atomic partices or high energy rays. This decay occurs at
a constant, predictable rate that is referred to as half-life. A stable
nucleus will not undergo this kind of decay and is thus non-radioactive.
Why are the nuclei of radioisotopes unstable? In order to answer this
question we must examine how the number of protons and neutrons in a
nucleus are related to its stability and how this relates to radioactive
The figure below shows a plot in which stable nuclei are positioned
according to the number of protons (Z) and the number of neutrons (A-Z)
that they contain. The stable (non-radioactive) nuclides are shown to
reside in the zone of stability. Nuclei of atoms that do not contain a
number of protons and neutrons that allows then to be plotted in this
region are unstable and they will spontaneously decay until a nucleus is
formed that does not reside in this stable zone.
Radioactive nuclei can undergo decomposition in a variety of ways. The
spontaneous decay process can produce particles as in the case of
alpha, beta, or positron emission. The alternate form of
emission is that of electromagnetic radiation such as x-rays or
When alpha, beta, or positrons are emitted from the nuclei of a
radioactive atom, it changes into a nucleus of another element.
Scientists refer to this as transformation. Emission of gamma rays
results only in a release of energy, not in transformation.
An alpha particle is simply a helium nuclei (He) which is ejected with
high energy from an unstable nucleus. This particle, which consists of
two protons and two neutrons, has a net positive charge. Although
emitted with high energy, alpha particles lose energy quickly as they
pass through matter of air and therefore, do not travel long distances.
They can even be stopped by a piece of paper or the outer layers of human
skin. These slow moving particles are generally the product of heavier
Beta particles are identical to electrons and thus have a charge of
(-1). This type of decay process leaves the mass number of the nuclei
unchanged. A beta particle is minute in comparison to that of an alpha
particle and has about one hundred times the penetrating ability. Where
an alpha particle can be stopped by a piece of paper a beta particle can
pass right through. It takes aluminum foil or even wood to stop a beta
particle. The electron that is released was not present before the decay
occured, but was actually created in the decay process itself.
Note that the mass number is unchanged and a new element is formed. So
what was the effect of this Beta particle production? It actually
changed a neutron into a proton. Notice that this new element will be
down and to the right on the zone of stability plot.
This type of particle production is just the opposite of Beta particle
Notice that is still has the same zero mass as an electron but an
opposite charge. This is what is known as an antiparticle of the
What happens when a positron collides with an electron?
As the name implies, these are not particles but high energy photons and
can be found on the electromagnetic spectrum. They are very similar to
x-rays but have a shorter wavelength and therefore more energy. The
penetrating ability of gamma rays is much greater than that of alpha or
beta particles. They can only be stopped by several centimeters of lead
or more than a meter of concrete. In fact, gamma rays can pass right
through the human body. Gamma rays often accompany other processes of
decay such as alpha or beta. An example of this was our previous
representation of an alpha particle process.
A ramification of alpha or beta particle production is that the newly
formed nucleus is left in a state of excess energy. A way for the
nucleus to release this excess energy is by emitting gamma rays. Since
gamma rays have no mass, and are waves rather than particles, the
elements atomic number does not change after emission.
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