Question

A sulphur-32 atom contains 16 protons, 16 electrons. What is the mass of sulphur-32/

Answer 1

The mass of an element is given by the sum of protons and neutrons present in it. So Sulphur-32 has a mass of 32 grams as it has 16 protons and 16 neutrons.


But 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)

In your case, the average atomic mass of sulfur will be calculated using the given atomic masses of its four isotopes and their respective decimal abundance, which is simply the percent abundance divided by 100.

So, you know that you have

32S: 31.972 u→95.002% abundance
33S: 32.971 u→0.76% abundance
34S: 33.967 u→4.22% abundance
36S: 35.967 u→0.014% abundance

This means that the average atomic mass of sulfur will be

avg. atomic mass=31.972 u×0.95002+32.971 u×0.0076+33.967 u×0.0422+35.967 u×0.00014

avg. atomic mass=32.063 u