Many nuclei are radioactive. This means they are unstable and will eventually decay by emitting a particle, transforming the nucleus into another nucleus or into a lower energy state. A chain of decays takes place until a stable nucleus is reached.

Nuclear reactions must satisfy conservation of energy, momentum, and charge, in addition to conserving nucleon number. The total number of nucleons (neutrons + protons) must be the same before and after a decay.

There are three common types of radioactive decay, alpha, beta, and gamma. The difference between them is the particle emitted by the nucleus during the decay process.