Page "Chemical element" Paragraph 19

The relative atomic mass ( historically and commonly also called " atomic weight ") of an element is the average of the atomic masses of all the chemical element's isotopes as found in a particular environment, weighted by isotopic abundance, relative to the atomic mass unit ( u ).

This number may be a fraction that is not close to a whole number, due to the averaging process.

For example, the relative atomic mass of chlorine is 35. 453 u, which differs greatly from a whole number due to being made of an average of 76 % chlorine-35 and 24 % chlorine-37.

Whenever a relative atomic mass value differs by more than 1 % from a whole number, it is due to this averaging effect resulting from significant amounts of more than one isotope being naturally present in the sample of the element in question.