## Chemistry: The Central Science (13th Edition)

$\frac{31}{16}X$ and $\frac{32}{16}X$ are isotopes of the same element. This element is Sulfur.
The identity of an element is determined by the number of protons it has. In the symbol $\frac{32}{16}X$, the lower number is the number of protons while the higher number is the atomic mass, which changes based on an isotope. However, because the question asks what are isotopes of the same element, the atomic mass does not matter. Rather, the only important thing is the number of protons. Because $\frac{31}{16}X$ and $\frac{32}{16}X$ have the same number of protons, they are the same element. Looking at the atomic number on the periodic table, it becomes clear that these are Sulfur isotopes as they both have 16 protons.