Since a nucleus contains protons and neutrons, most of the mass of an atom is concentrated in its nucleus.

Elements with lower and higher mass numbers per nucleon are less stable. The total mass of a nucleus is less than the total mass of the nucleons that make up the nucleus. This difference is known ...

As they studied atomic disintegration, they kept seeing that the atomic number (number of protons in the nucleus, equivalent to the positive charge of the atom) was less than the atomic mass ...