Radioactivity is the spontaneous emission of particles or energy from an unstable atomic nucleus. The modern atomic theory pictures the nucleus as protons and neutrons held together by a short-range nuclear force that has moving nucleons (protons and neutrons) in energy shells analogous to the shell structure of electrons. A graph of the number of neutrons to the number of protons in a nucleus reveals that stable nuclei have a certain neutron-to-proton ratio in a band of stability. Nuclei that are above or below the band of stability, and nuclei that are beyond atomic number 83, are radioactive and undergo radioactive decay.

Three common examples of radioactive decay involve the emission of an alpha particle, a beta particle, and a gamma ray. An alpha particle is a helium nucleus, consisting of two protons and two neutrons. A beta particle is a high-speed electron that is ejected from the nucleus. A gamma ray is a short-wavelength electromagnetic radiation from an excited nucleus. In general, nuclei with an atomic number of 83 or larger become more stable by alpha emission.Nuclei with a neutron-toproton ratio that is too large become more stable by beta emission. Gamma ray emission occurs from a nucleus that was left in a highenergy state by the emission of an alpha or beta particle.

Each radioactive isotope has its own specific radioactive decay rate. This rate is usually described in terms of half-life, the time required for one-half the unstable nuclei to decay.

Radiation is measured by (1) its effects on photographic film, (2) the number of ions it produces, or (3) the flashes of light produced on a phosphor. It is measured at a source in units of a curie, defined as 3.70 × 10¹⁰ nuclear disintegrations per second. It is measured where received in units of a rad, defined as 1 × 10^-5J. A rem is a measure of radiation that takes into account the biological effectiveness of different types of radiation damage. In general, the natural environment exposes everyone to 100 to 500 millirems per year, an exposure called background radiation. Lifestyle and location influence the background radiation received, but the average is 130 millirems per year.

Energy and mass are related by Einstein's famous equation of E = mc², which means that matter can be converted to energy and energy to matter. The mass of a nucleus is always less than the sum of the masses of the individual particles of which it is made. This mass defect of a nucleus is equivalent to the energy released when the nucleus was formed according to E = mc². It is also the binding energy, the energy required to break the nucleus apart into nucleons.

When the binding energy is plotted against the mass number, the greatest binding energy per nucleon is seen to occur for an atomic number near that of iron. More massive nuclei therefore release energy by fission, or splitting to more stable nuclei. Less massive nuclei release energy by fusion, the joining of less massive nuclei to produce a more stable, more massive nucleus. Nuclear fission provides the energy for atomic explosions and nuclear power plants. Nuclear fusion is the energy source of the sun and other stars and also holds promise as a future energy source for humans.

Summary of Equations