Some atoms possess nuclei that are unstable. To achieve stability, these unstable nuclei spontaneously emit energy (radiation). Such atoms are said to be radioactive.

The types of radiation emitted by naturally occurring radioactive nuclei are alpha, beta, and gamma. These radiations can be characterized by mass and charge values. Alpha particles carry a positive charge, beta particles carry a negative charge, and gamma radiation has no charge.

The procedures for balancing nuclear equations are different from those for balancing ordinary chemical equations. In nuclear equations, mass numbers and atomic numbers (rather than atoms) balance on both sides.

Every radionuclide decays at a characteristic rate given by its half-life. A half-life is the time required for half of any given quantity of a radioactive substance to undergo decay.

Nuclear fission is the splitting of a large nucleus into two smaller nuclei and one or more neutrons. When the free neutrons are captured efficiently by other nuclei, a chain reaction can occur.

Nuclear reactors use the heat from a controlled nuclear fission reaction to produce power.

Nuclear fusion, the type of reaction that occurs in the sun, is the combination of two light nuclei to form one heavy nucleus. Fusion takes place only at very high temperatures.

Radioactive isotopes are easy to detect and thus make excellent tracers in chemical reactions and in medical practice.

The biochemical effects of radiation depend on the energy, ionizing ability, and penetrating ability of the radiation. Alpha particles exhibit the greatest ionizing effect, and gamma rays have the greatest penetrating ability. High-energy radiation damages living systems by causing ionization and the formation of free radicals.