Radioactive decay is the process where large unstable atoms, like Uranium, break down into smaller ones. Radioactive decay was first investigated by Marie Curie.
The radioactive decay of an individual atom is completely random. It is impossible to predict when it will happen.
However the probability of an atom decaying is not random at all—it is constant. This makes it possible to work out the number of atoms that will decay-although it is impossible to predict exactly which ones.
There are many different radioactive elements, like Uranium, Plutonium and Radium. These elements all decay in a similar way. The only difference is the length of time involved. Scientists compare different radioactive elements by comparing a number called the "half-life". This is the time it takes for half the atoms in a sample to decay. After one half-life, a half of the original atoms will be left. After two half lives, half of the remaining half (one quarter of the original atoms) will be left, and so on.
An element with a short half-life will decay very quickly, emitting a lot of radiation. An element with a long half-life will decay slowly.
Radioactive decay is not necessarily a one-step process. Uranium-238, for example, goes through many changes before the stable form, Lead-206, is reached.