The atomic number is the number of protons per atom. In a neutral atom, the number of electrons equals the number of protons. The periodic table represents neutral atoms. ... The atomic mass number is equal to the number of protons plus neutrons, which provides the average weight of all isotopes of any given element. Isotopes are atoms of the same element that have different masses. Atoms have a mass that is based largely on the number of protons and neutrons in their nucleus. Electrons are organized into shells and subshells about the nucleus of an atom. The chemical elements are arranged in a chart called the periodic table.