what makes something radioactive

Radioactivity is the release of energy from the decay of the nuclei of certain kinds of atoms and isotopes. Atoms found in nature are either stable or unstable. An atom is stable if the forces among the particles that make up the nucleus are balanced. An atom is unstable (radioactive) if these forces are unbalanced; if the nucleus has an excess of internal energy. Radioactive nuclei are nuclei that are unstable and that decay by emitting energetic particles such as photons, electrons, neutrinos, protons, neutrons, or alphas (two protons and two neutrons bound together). Some of these particles are known as ionizing particles, which are particles with enough energy to knock electrons off atoms or molecules. The degree of radioactivity depends on the fraction of unstable nuclei and how frequently those nuclei decay. The effect of radioactivity also depends on the type and energy of the particles produced during nuclear decay. Radioactivity can cause damage in materials and in plant, animal, and human tissue.