Oxidation numbers assigned to Cl in HClO4 and ClO3

Question

I need help please. I don't understand this

Answer 1

H is always 1+ except in hydrides.O is 2- except in peroxides. So in HClO₄ you have 1+ from H and 8- from the 4 oxygens. That means Cl must be 7-.

 

In ClO₃ you have 6- from the 3 oxygens and if you mean the chlorate anion ClO3^-, then Cl will have an oxidation number of 5+. If you really mean ClO₃, then it would have to be 6+.