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 .
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