Fission: The breaking down of a Nucleus (not atom) into

smaller nuclei. It is usually induced by a neutron.

For example, a Helium nucleus (called alpha particle) is

divided into two 4He(+2) --> 2H(+1) + 2H(+1)

A lot of enery is released in the process.

Fusion: This happens when two nuclei combines to form a

larger nuclei. Huge amount of enery is needed to start

this. Because its not easy to bring two positively charged

nuclei closer.

When they combine, a huge amount of energy is released.

This usually happens in the stars.

The enery required to start the fusion comes from the

gravitational force between the particles.