The average atomic mass of a chemical element is calculated by taking into account the atomic masses of its naturally occuring isotopes and their respective abundances.
avg. atomic mass=∑i(isotopei×abundancei)
atomic masses of four isotopes of sulphur are
32S: 31.972 u→95.002%
33S: 32.971 u→0.76%
34S: 33.967 u→4.22%
36S: 35.967 u→0.014%
the average atomic mass of sulfur will be
Average atomic mass=31.972 u×0.95002+32.971 u×0.0076+33.967 u×0.0422+35.967 u×0.00014
Average atomic mass=32.063 u
