Atoms become radioactive when they have an excess of protons or neutrons in the nucleus, leading to unbalanced internal forces, which the atom balances by emitting radiation. Atoms with a different amount of neutrons or protons from their normal configuration are called ions and are an isotope of their element.
Atoms become stable by reconfiguring into a new nucleus and releasing energy as particles or radiation, which is radioactive decay. It occurs at a steady rate called a half-life, or the time it takes for half of a given sample of a radioactive isotope to decay into a new isotope.
Radioactive decay involves the emission of alpha or beta particles, gamma rays, or by other processes. Alpha particles are high-energy helium nuclei. Beta particles are essentially electrons. Gamma rays are energetic, electromagnetic photons with a short wavelength. Other forms of radioactive decay include electron capture and positron decay.