Please explain "The density of nuclear matter is independent of size of nucleus but the mass density of the atom does not follow this rule".

Every nucleus is made of same elementary particles i.e. protons and neutrons and they're all bound by same type of attraction power, so the compactness with which they're packed doesn’t depend on the number. It’s like if I were to take 2 g of water or 5 g of water, its density is still the same as water is made of same type of molecules. However when we talk about matter, matter's made up of atoms which in turn is made up of neutron , proton and nucleus, and the number of protons does play crucial role in determining properties of element in the periodic table. Thus every element has different density but as nucleus everywhere is made of same thing its density remains the same.