Radioactive Decay

Radioactive decay is the process of the random, spontaneous transformation of a radionuclide by the emission of nuclear radiation

It is 'spontaneous' because we cannot 'make it happen' by changing the conditions the sample is in - such as making it hotter or putting it under pressure. Similarly we cannot stop it happening - that is why nuclear waste is such a problem!

The emission of the nuclear radiation is a purely random event. It cannot be predicted exactly when an atom will decay, only that a certain number will decay in a given time. The mathematics of probability is used for this requires a large number of atoms to be considered. (See half life and radioactive decay series).

Radioactive nuclear decay occurs whenever a nucleus is in an energy-state that is not the lowest possible for its nucleon number. This state may occur naturally (which essentially means that it was created in that state when formed within a star) or by artificial means (neutron or photon irradiation).

The nucleus remaining is called the decay product or daughter nucleus

The rate of decay depends on the number of undecayed nuclei present, so with each decay event there is a decrease in the activity of a radioactive sample.

See here for the five types of nuclear radiation.

See here for the dangers of nuclear radiation.

For a more mathematical look at this - see here