The nucleus of every atom is composed of massive particles: #"protons"#, with a positive electrostatic charge; and #"neutrons"#, the which are neutrally charged. The NUMBER of protons DEFINES the atomic number absolutely...and #Z_"the atomic number"# DEFINES the identity of the element:
#"Z = 1, hydrogen; Z = 2, helium; ..... Z = 92 uranium"#
The number of neutrons, defines the identity of the isotope.. For protium, #""^1H#, there is one nuclear particle, the defining proton. For deuterium, #""^2H#, there are TWO nuclear particles, the defining proton, and a neutron. For tritium, #""^3H#, there are THREE nuclear particles, the defining proton, and TWO neutrons.
As #Z# increases, isotopic substitution becomes more common, and the atomic mass quoted on the Periodic Table is the weighted average of the individual isotopes.
