Average Atomic Mass: Determination, Examples

Average Atomic Mass

“The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance. Atomic Masses depends upon the number of possible isotopes and their natural abundance.”


Neon isotopes are 1020Ne , 1021 Ne , 1022 Ne in the percentages of 90.92% , 0.26% , 8.82% respectively. Now we will calculate the atomic mass of Neon.
The overall atomic mass of neon, which is the ordinary isotopic mixture, is the average of the determined atomic masses of individual isotope. Hence
Average Atomic Mass = 20×90.92 + 21×0.26 + 22×8.82 = 20.18 amu
Hence the average atomic mass of Neon is 20.18 amu.


It is important to realize that no individual neon atom in the sample has a mass of 20.18 amu. For most laboratory purposes, however, we consider the sample to consist of atom with this average mass

Atomic Masses of some Elements

Elements Average atomic mass (amu)
Carbon 12.01 amu
Hydrogen 1.008 amu
Oxygen 15.999 amu
Aluminium 27 amu
Neon 20.18 amu
Uranium 238.029 amu
Chlorine 35.453 amu

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