All naturally occurring elements with atomic numbers greater than 83, as well as some isotopes of lighter elements, are radioactive. Based on later work by Rutherford, Soddy, Villard, and others, three different types of radiation were identified. Alpha particles are helium nuclei, containing two protons and two neutrons. They are deflected slightly in an electric of magnetic field. Their penetrating power is very low, being stoppable by a thin sheet of aluminum or paper.
Beta particles are electrons capable of travelling at speeds approaching the speed of light.
Their low mass allows them to be deflected greatly in an electric or magnetic field, in the opposite direction as the deflection of alpha particles. Their high speed gives them greater penetrating power than alpha particles. Some beta particles can penetrate several centimetres of aluminum. Alpha particle emissions and beta particle emissions change the composition of the nucleus. Gamma rays are high energy electromagnetic radiation with short wavelengths.