Answer to Question #139265 in General Chemistry for Isurini Sirisena

Question

2. Explain why the relative atomic masses of many elements are not exact whole numbers.

Answer 1

All isotopes of all elements have atomic masses that are essentially whole numbers. The non-integral atomic masses result from averaging the masses of the various isotopes weighted in proportion to their existence in nature.

For example Chlorine has two main isotopes Cl35 and Cl37, which occur in nature approximately in the proportion 76% and 24% respectively, giving an atomic mass of 35.45.