Radioactivity is caused by emission of high energy radiation from unstable atomic nuclei.
An atomic nucleus will be unstable if it has an excess of energy caused by an excess of either neutrons or protons.
The nucleus will decay into a more stable atom (usually a different element) from the parent atom.
An atom may decay through several states before reaching stability – this is known as the radioactive decay chain.
Radioactivity refers to the rate at which an unstable nucleus emits radiation. The rate is the half life , the time it takes one half of a sample of atoms to convert to a more stable atom.
Half lives can range from less than one second to billions of years .
One unstable element is Uranium.
Nuclear fission is a human induced process that involves splitting a Uranium isotope 235 atom with a thermal electron into Barium isotope 141 and Krypton isotope 92 (themselves radioactive).
There is a resulting release of energy and three more neutrons , causing a nuclear chain reaction .