What is the formal oxidation number for the Cl atom in ClO4 ^1-?

Answer 1

First, look at the oxygen. Oxygen has a charge of 2-, and there are 4, so in the entire molecule, oxygen gives a charge of 8-. The total charge of the molecule is 1-, therefore to find Cl, create the equation -8+x = -1
x=7
Therefore chlorine has a charge of 7+
The total charge of the molecule = the sum of the charges of each atom

Answer 2

+3.