how to find oxidation number?

Question

how would one work out the oxidation number or Chlorine in NH4ClO4?

Answer 1

There are certain rules to follow to determine the oxidation number of different atoms.
1) The oxidation number of an element not joined to another is always 0.
2) The alkali metals are always 1+ (group 1A)
3) Alkaline earth metals are almost always 2+ (group 2A)
4) Oxygen is always 2- unless it is a peroxide. Then it is 1-
5) Hydrogen is almost always 1+ unless it is a hydride. Then it is 1-.

Your problem:

NH4ClO4 needs to be divided into cation and anion.

cation is NH4 1+, anion is ClO3 1-

You need the oxidation number for Cl, so we will use only the ClO3 1-.

Oxygen is not in a peroxide. It is in "chlorate" so the oxidation number of oxygen is 2-. There are 3 oxygens in the ion. 2- x 3 = 6- total. The chlorate ion has a 1- charge so there are 5- charges to be balanced out by positive charges. That means that the Cl (the only other atom there) has to balance out the 5- left over by having a 5 + oxidation number.

(Cl 5-) + (O3 6-) = ClO3 1-