Question

How can you find an oxidation number?

Answer 1

You can work them out based on the following rules for oxidation states.

1. All uncombined elements have an oxidation number of zero:
   eg. Na, `O_2`, `F_2`

2. Compounds are always neutral overall: so all oxidation numbers in it add to zero:
   eg. NaCl is neutral.
   Na is Na+ and Cl is Cl-

3. Some elements always have the same oxidation state:

   • Group 1 are +1
   • Group 2 are +2
   • Aluminium is +3
   • Fluorine is -1

4. Others can be tricksy based on the compounds they are in:

   • Hydrogen always +1 except in metal hydrides (like NaH) where it is -1
   • Oxygen always -2, except in peroxides, superoxides, and `OF_2`