To determine the valency of sulfur, we need to know the valencies of iron and the overall charge of the compound. Iron (Fe) has a valency of 2+ in FeS2. Since the compound is neutral, the sum of the valencies of iron and sulfur must be equal to zero:
2+ + 2(x) = 0
2(x) = -2
x = -1
Therefore, the valency of sulfur in FeS2 is -1.
