We have to use the periodic table to determine whether the fourth isotope of sulfur is sulfur-32 (32S) or sulfur-35 (35S).
The other three sulfur isotopes are sulfur-33 (33S), sulfur-34 (34S) and sulfur-36 (36S). We will first have a look at the formula for average atomic mass of an element.
M = isotopic mass of an isotope
FA = fractional abundance of an isotope
As we know that the average atomic mass is the weighted average of all isotope masses, the average atomic mass can give us some clues about the isotopic composition of an element.
Sulfur has four naturally occurring isotopes, including 33S, 34S, and 36S. Look at the periodic table, and tell whether the fourth isotope is 32S or 35S.