Radioactive decay is the process by which a radioactive atom emits particles and energy to reach a more stable configuration. Atoms may emit alpha or beta particles from the nucleus or may spontaneously split into different isotopes, producing gamma or neutron radiation.
When an atom undergoes alpha decay, it ejects two protons and two neutrons bound together. Beta decay involves the transformation of a neutron into a proton, electron and antineutrino, and it ejects the electron and antineutrino. In spontaneous fission, the nucleus fragments into two different clusters of protons and neutrons, forming two new atoms. Spontaneous fission usually results in the release of spare neutrons, which can cause other atoms to split.
The particles and rays emitted by elements undergoing radioactive decay are extremely dangerous to living things. Alpha particles cannot penetrate skin or clothing, but if an atom emitting alpha rays is ingested, it damages tissue from inside the body. Beta particles can penetrate skin and do more damage if taken internally. Gamma radiation is much more powerful and can only be blocked by dense materials, such as thick sheets of lead. Neutron radiation can actually induce radioactivity in other materials by altering the atomic structure of any atoms it encounters. The radioactive fallout from a nuclear weapon consists of ordinary soil, ash and debris rendered radioactive by neutron radiation.