# Why isn't mass number listed with each element on the periodic table?

The Periodic Table lists elements according to $Z$, the atomic number, which represents the number of protons, massive, positively charged nuclear particles. The number of protons, i.e. $Z$, determines atomic identity, $Z$ $=$ $1$, $\text{H}$; $Z$ $=$ $2$, $\text{He}$; $Z$ $=$ $23$, $\text{V}$, etc.
Each atomic nucleus may contain varying number of NEUTRONS, massive, neutrally charged nuclear particles. Different numbers of neutrons gives rise to the existence of isotopes: most hydrogen nuclei have only 1 proton in their nuclei, the protium isotope, ""^1H, a smaller number have the 1 proton (necessarily) but also 1 neutron to give the deuterium isotope, ""^2H, an even smaller number of hydrogen nuclei have 2 neutrons in their nuclei to give the tritium isotope, ""^3H.