Radioactivity is when a certain kind of atom disintegrates spontaneously and sends out little bursts of radiation from its nucleus (center).
Isotopes are slightly different versions of an atom, with either more or less neutrons. With stable elements, such as carbon, only certain isotopes called radio-isotopes are radioactive.
Some large atoms, such as radium and uranium, are so unstable that all their isotopes are radio-isotopes.
Radioactive isotopes emit three kinds of radiation: alpha, beta and gamma rays.
When the nucleus of an atom emits alpha or beta rays it changes and becomes the atom of a different element. This is called radioactive decay.
Alpha rays are streams of alpha particles. These are made from two protons and two neutrons — basically the nucleus of a helium atom. They travel only a few centimeters and can be stopped by a sheet of paper.
Beta rays are beta particles. Beta particles are electrons (or their opposite, positrons) emitted as a neutron decays into a proton. They can travel up to 1 m and can penetrate aluminum foil.
Gamma rays are an energetic, short-wave form of electromagnetic radiation. They penetrate most materials but not lead.
The half-life of a radioactive substance is the time it takes for its radioactivity to drop by half. This is much easier to assess than the time for the radioactivity to disappear altogether.