An atom has a central, dense, tightly packed nucleus with a diameter of 10-15 m compared to an atom at 10-10 m, a size equivalent to a marble in a football stadium. Therefore, the nucleus makes up less than 0.01 % of the atom's volume.
The nucleus contains positively charged protons and neutral neutrons as subatomic particles bound firmly by the nuclear force.
The nuclear entities- the protons and the neutrons- are responsible for contributing 99.94 % of the atom's mass. Another atom's component, the electrons situated in the outer-nuclear region, has negligible mass.
The proton number identifies an atom, whereas the neutrons are responsible for an atom to exist in more than one form as the isotopes.
For example, proton number 6 is a Carbon atom. When the Carbon atom also has 6 neutrons, it occurs as the most abundant 12C form (98.89 %). Increasing the neutrons to 7 and 8 gives 13C and 14C isotopes with a natural abundance of only 1.11 % and 1 part per trillion.