Why are some atoms radioactive?

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.