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

1 Answer
anor277
Feb 24, 2016

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.

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