We have to use the periodic table to determine whether the fourth isotope of sulfur is sulfur-32 (^{32}S) or sulfur-35 (^{35}S).

The other three sulfur isotopes are sulfur-33 (^{33}S), sulfur-34 (^{34}S) and sulfur-36 (^{36}S). We will first have a look at the **formula for average atomic mass of an element**.

$\overline{){\mathbf{Avg}}{\mathbf{.}}{\mathbf{}}{\mathbf{atomic}}{\mathbf{}}{\mathbf{mass}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}\mathbf{(}{\mathbf{M}}_{\mathbf{1}}\mathbf{\times}{\mathbf{FA}}_{\mathbf{1}}\mathbf{)}{\mathbf{}}{\mathbf{+}}{\mathbf{}}\mathbf{(}{\mathbf{M}}_{\mathbf{2}}\mathbf{\times}{\mathbf{FA}}_{\mathbf{2}}\mathbf{)}}$

**Where,**

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 ^{33}S, ^{34}S, and ^{36}S. Look at the periodic table, and tell whether the fourth isotope is ^{32}S or ^{35}S.