Why isn't mass number listed with each element on the periodic table?
The Periodic Table lists elements according to
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,
The weighted average of the individual isotopes gives the atomic mass of the element, as listed on the Periodic Table. Most elements have a number of isotopes, and their weighted average reflects the individual isotopic masses.