To calculate the formal charge on the central atom Sulfur in the compound SO$_3$, follow these steps:Step 1: Determine the number of valence electrons for the central atom Sulfur .Sulfur is in group 16 or 6A of the periodic table, so it has 6 valence electrons.Step 2: Calculate the number of bonding electrons.In SO$_3$, there are three S-O bonds. Each bond consists of 2 electrons, so there are a total of 3 x 2 = 6 bonding electrons.Step 3: Calculate the number of non-bonding electrons.Since Sulfur has 6 valence electrons and all of them are used in bonding, there are 0 non-bonding electrons.Step 4: Calculate the formal charge.Formal charge = valence electrons - 1/2 x bonding electrons - non-bonding electrons Formal charge = 6 - 1/2 x 6 - 0Formal charge = 6 - 3Formal charge = 3Therefore, the formal charge on the central atom Sulfur in the compound SO$_3$ is +3.