The nuclei of the atoms in some elements are unstable and emit radiation. They are said to be radioactive.
The radiation is of three main types. We now know what these radiations are, but the French scientist Becquerel who first discovered radioactivity didn't know, and named the three types of radiation with three letters of the Greek alphabet: α (alpha), β (beta) and γ (gamma).
We now know that :
Radioactive nuclei carry on giving out these radiations whatever is happening in their surroundings, e.g. chemical reactions or change of temperature. The radiations transfer energy from the nucleus and ionise the substances they pass through.
Radioactive elements are found naturally in the environment. The radiation these give off is called background radiation.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science