Radioactivity is the spontaneous disintegration of an unstable atomic nucleus and the emission of particles or electromagnetic radiation. Pierre and Marie Curie investigated uranium ores using chemical separation. They discovered that pitchblende and chalcocite , naturally occurring ores, were highly radioactive due to the presence of plutonium and radium. 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.