CBSE Class 10 Answered

Nuclear fusion:

In nuclear fusion reactions, two light nuclei are combined to form a heavier and a more stable nucleus. When nuclear fusion occurs, large amount of energy is released.

The example of nuclear reaction is the reaction which takes place in the Sun.

We can see that the element formed has more protons or more neutrons than that of the starting material. 

Nuclear fission:

It is the reverse process. In fission reactions, a heavy nucleus is split into two nuclei with smaller mass numbers when bombarded with low energy neutrons. A tremendous amount of energy is produced in this process. So, there is a small loss in the mass in form of tremendous amount of energy.

The three neutrons formed can collide with other U-235 atoms, setting off a chain reaction and releasing tons of energy.