he formal charge over an atom of a polyatomic molecule or ion is the difference between the valence electron of that atom in the elemental state and the number of electrons assigned to that atom in Lewis structure.
Formal Charge on Cl atom = [No. of valence electrons on atom] – [non-bonded electrons + number of bonds] =7-(1/2) x (8)-0=3
= 7 – 4 = 3
Formal Charge on Cl in HClO4= 3