Step-by-step explanation:
It is known that protons are positively charged and an they reside inside the nucleus of an atom. So, due to like charges protons repel each other and this force is known as electrostatic force of repulsion.
But a strong force that binds the protons and neutrons together is called nuclear binding energy.
Therefore, when there is large difference in the number of protons and neutrons then electrostatic force of repulsion overcomes the nuclear binding energy causing the atom to become unstable.
For example,
is an unstable atom as it has 88 protons and 138 neutrons. Hence, this atom will decay.