Radioactive decay is a process whereby unstable nuclei change into more stable nuclei by emitting particles of different kinds. Alpha, beta and gamma (α, β and γ) radiation was classified according to its different penetrating power.

• An α particle is a 4He nucleus, and is emitted by some heavy nuclei, producing a nucleus with Z two units less and mass number four units less. For example, 238U (Z=92) undergoes a decay to produce (radioactive) 234Th (Z=90).
• A β particle is an electron. Its emission by a nucleus increases Z by one unit, but does not affect the mass number. Thus 14C (Z=6) decays to (stable) 14N (Z=7).
• γ radiation have high-energy electromagnetic radiation. It accompanies α and β decay.