What occurs during nuclear decay? and What is the concept of half-life?

Answer 1

If you have 100 atoms of a certain radioactive element (say uranium), the half life is the time that it takes for the 100 to become 50.

Answer 2

In nuclear decay atoms eject pieces, particles and rays in order to become more stable.

The half-life is how long it will take for a sample to undergo decay to half of the original amount. Some isotopes decay in trillionths of a second, others take a billion years or more, however all atoms, once they begin to decay, do so in a very tiny fraction of a second.