A radioactive substance is one which has an unstable nucleus and gives out radiation to try to become stable. This is also known as radioactive decay. Radioactivity is a completely random process. This means that you cannot predict when an unstable nucleus will decay. It's completely unaffected by physical conditions such as temperature.
Radioactivity is defined as the spontaneous disintegration or decay of some elements with the emission of alpha, beta and gamma radiations.
Radioactive elements are elements that disintegrate by emitting radiations capable of ionising air molecules or any matter through which they pass. There are two types of radioactive elements – natural and artificial radioactive elements. The natural radioactive elements include uranium, radium, thorium, Plutonium. The artificial radioactive elements are cobalt-60, iodine-131, and sulphur-35.