Rules to Remember:
* Oxygen (O): Usually has an oxidation number of -2, except in peroxides (like H2O2) where it is -1.
* Hydrogen (H): Usually has an oxidation number of +1, except in metal hydrides (like LiH) where it is -1.
* The sum of oxidation numbers in a neutral compound is zero.
* The sum of oxidation numbers in a polyatomic ion equals the charge of the ion.
Applying the Rules to HClO4:
1. Oxygen (O): There are four oxygen atoms, each with an oxidation number of -2, for a total of -8.
2. Hydrogen (H): There is one hydrogen atom with an oxidation number of +1.
3. Chlorine (Cl): Let the oxidation number of chlorine be 'x'.
4. Sum: The sum of the oxidation numbers must equal zero since HClO4 is a neutral compound: x + (+1) + (-8) = 0
5. Solving for x: x - 7 = 0
Therefore, x = +7
Therefore, the oxidation number of chlorine (Cl) in HClO4 is +7.
