A natural sample of an element is a mixture of isotopes with different mass numbers. The average mass of these atoms in a naturally occurring sample is called the relative atomic mass of the element.
Thus we can obtain the relative atomic mass of chlorine known to have chlorine -35 and chlorine -37 isotopes occurring in the relative abundance of and respectively. Generally, the relative atomic mass of an element that is composed of isotopes A and B with relative abundance of a and b respectively is given by the expression:
where A is the mass number of isotope A and B is the mass number of isotope B. For chlorine therefore: