In the compound Na2SO4, the valency of Sulfur (S) can be calculated by looking at the charges of the other elements in the compound.

First, we need to determine the valency of Sodium (Na). Sodium is in Group 1 of the periodic table, and elements in this group generally have a valency of +1. Therefore, since there are two sodium atoms in the compound, the total valency of Sodium in Na2SO4 is +2.

Next, we need to calculate the valency of Oxygen (O). Oxygen is in Group 16 of the periodic table, and elements in this group generally have a valency of -2. Since there are four oxygen atoms in the compound, the total valency of Oxygen in Na2SO4 is -8.

To find the valency of Sulfur (S), we can subtract the valencies of Sodium and Oxygen from the overall charge of the compound. The compound has a neutral charge, so the sum of all the valencies should equal zero.

2 (valency of Sodium) + x (valency of Sulfur) + 4 (valency of Oxygen) = 0

Simplifying the equation, we get:

2 + x -