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.
