All oxygen atoms have 8 protons and all sodium atoms have 11 protons. The nucleon number (or mass number) of an atom is the total number of protons and neutrons it contains. The nucleon number of ...

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

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 ...