The force with which two protons, two neutrons, or a proton and a neutron, attract one another in the nucleus is called nuclear force There are two types of particles in the nucleus of an atom: protons and neutrons, which taken together are called nucleons. Nuclei of small atoms are quite stable but the nuclei of big atoms are quite unstable. In order to know the reason, let us first discuss the various forces which operate inside the nucleus of an atom where we have positive protons and neutral neutrons.

There are two types of forces operating inside the nucleus of an atom. These are:

1. Electrostatic Force

The force which a charged particle exerts on another charged particle is called electrostatic force. If two particles have opposite charges, then the electrostatic force will cause attraction between them but if the particles have similar charges, then the electrostatic force will be repulsive and cause repulsion between the similarly charged particles.

As, the nucleus of an atom contains only positively charged protons, so in the nucleus of an atom, the electrostatic force is of repulsive nature and hence causes repulsion between the various protons present in the nucleus.

Therefore, if there had been only electrostatic force operating in the nucleus, then no nucleus of any atom would have been stable. The stability of the nucleus comes from the operation of another type of force in the nucleus called nuclear force or strong force.

2. Nuclear Force (or Strong Force)

The force with which two protons, two neutrons, or a proton and a neutron, attract one another in the nucleus is called nuclear force (or strong force) . Irrespective of the nature of charges of the particles involved, the nuclear force (or strong force) is always an “attractive force” which causes attraction between the particles.

Thus, the nuclear force can cause attraction even between a proton and another proton, even though they are similarly charged (both having positive charge). A nuclear force can also cause attraction between a proton and a neutron, as well as between a neutron and another neutron.

One important characteristic of nuclear force (or strong force) is that it is effective only when the particles like protons and neutrons are very, very close together. In other words, nuclear forces are short range forces. Strong force only operates at EXTREMELY small distances. These distances are on the order of a 1000th millionth millionth of a meter (10 to the power of -15). If you think about a micrometer (one hundredth the size of a human hair), it is a billion times smaller than that.

Thus, due to these two types of forces, we can explain that why the nucleus of small atoms is stable whereas that of big atom is unstable.

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