CK-12-Chemistry Intermediate

24.1. Nuclear Radiation http://www.ck12.org

There are several other characteristics common to stable nuclei. For example, stable nuclei with even numbers of
both protons and neutrons are far more common than stable nuclei with odd numbers of one or both of those particles
(Table24.1).

TABLE24.1:Isotopes

Protons Neutrons Number of Stable Isotopes

Odd Odd 4

Odd Even 50

Even Odd 53

Even Even 157

Nuclei that contain 2, 8, 20, 50, 82, or 126 protons or neutrons are unusually stable for a nucleus of that particular
size. For example, there are ten stable isotopes of the element tin (atomic number 50), but only two stable isotopes
of antimony (atomic number 51). These numbers are often referred to asmagic numbers, and their existence lends
evidence to the theory that nucleons, like electrons, exist in different energy levels within the nucleus. These numbers
are thought to represent filled energy levels, and their stability is analogous to the stable electron configurations of
noble gases.

Radioactive Decay

Unstable nuclei spontaneously emit radiation in the form of particles and energy. This generally changes the number
of protons and/or neutrons in the nucleus, resulting in a more stable nuclide. Anuclear reactionis a reaction that
affects the nucleus of an atom. One type of a nuclear reaction isradioactive decay,a reaction in which a nucleus
spontaneously disintegrates into a slightly lighter nucleus, accompanied by the emission of particles, energy, or both.
An example is shown below, in which the nucleus of a polonium atom radioactively decays into a lead nucleus.

210

84 Po→

206

82 Pb+

4

2 He

Note that in a balanced nuclear equation, the sum of the atomic numbers and the sum of the mass numbers must be
equal on both sides of the equation. Recall the notation system for isotopes, which shows both the atomic number
and mass number along with the chemical symbol.

Because the number of protons changes as a result of this nuclear reaction, the identity of the element changes.
Transmutationis a change in the identity of a nucleus as a result of a change in the number of protons. There are
several different types of naturally occurring radioactive decay, and we will examine each separately.

Alpha Decay

Analpha particle (α)is a helium nucleus with two protons and two neutrons. Alpha particles are emitted during
some types of radioactive decay. The net charge of an alpha particle is 2+, and its mass is approximately 4 amu.
The symbol for an alpha particle in a nuclear equation is usually^42 He , though sometimes^42 ff is used. Alpha decay
typically occurs for very heavy nuclei in which the nuclei are unstable due to large numbers of nucleons. For nuclei