The positive charge and entire mass of atom is concentrated at its centre called nucleus.
Radius of atom ≈10⁻¹⁰ m
Radius of Nucleus ≈10⁻¹⁵ m
Composition of Nucleus:
The nucleus was first discovered in 1911 by Lord Rutherford and his Associates by experiments on scattering of α-particle by atoms.
Nucleus is made up of photons and neutrons these are called nucleons and electron are revolving around the nucleus.