The nucleus of an atom contains positively charged particles, called protons, and neutral particles, called neutrons. how is the nucleus of an atom held together?

Explanation:

In a large nucleus there will be large number of protons and neutrons. Since, charge on protons is positive and neutrons have no charge. So, due to the like charges of protons there will be force of repulsion between them.  

As there are large number of protons as compared to the neutrons so, force of repulsion will also be large. Whereas a force which is responsible for holding the protons and neutrons together in a nucleus is known as nuclear binding energy.

Hence, when nuclear force of repulsion overcome the nuclear binding energy then the atom becomes unstable in nature.

If an atom is smaller then there will be less number of protons and neutrons. Hence, then nuclear binding energy overcomes the nuclear force of repulsion. Therefore, atom remains stable in nature.