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

Question

Cameron Andrews · Accepted Answer

The Periodic Table lists AVERAGE atomic mass.

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 atomic mass of an element can be found on the Periodic Table by calculating the weighted average of its individual isotopes. Since most elements have multiple isotopes, the weighted average of those isotopes represents the individual isotopic masses.

Luna Chen · Answer

undefined The mass number isn't listed on the periodic table because it represents the average mass of isotopes, which vary.