define nucleus of atom

Solution : 

The positively-charged central part of an atom, which constitutes almost all its mass. Except for hydrogen nuclei, which have only one proton.
Nuclei are composed of both protons and neutrons. Surrounding the nucleus are electrons, of equal and opposite charge to that of the protons, thus giving the atom a neutral charge. Nuclei that are unstable may undergo radioactive decay or nuclear fission. In all stars, including our Sun, small nuclei join together to make more stable, larger nuclei. This process is called nuclear fusion.

The nucleus was discovered by the New Zealand-born British physicist Ernest Rutherford in 1911 as a result of experiments in firing alpha particles through very thin gold foil.