Question

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

Answer 1

The Periodic Table lists AVERAGE atomic mass.

Explanation:

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`, `"H"`; `Z` `=` `2`, `"He"`; `Z` `=` `23`, `"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`.

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.