Assign oxidation numbers to the following compounds, elements or ion a) AI

The oxidation state of any element in its free (by itself or with atoms of same element) state is ##0##.

To start off, I'll write the rules for assigning :

1. The oxidation state of any element in its free state is ##0##.
2. The oxidation state of monatonic (one-atom) ion is equal to the charge on that ion.
3. In a neutral polyatomic molecule, the sum of the oxidation numbers of the atoms must equal zero.
4. In a polyatomic ion, the sum of the oxidation numbers must equal to the charge on that ion.
5. The oxidation number of oxygen is always -2 except in hydroxides in which the oxidation number is -1. ... There are few more major rules, but I'll only write 5 not to overwhelm you.

Since the element Aluminium is by itself, the rule number 1 applies. The oxidation number on it is ##0##.

However, if the aluminium was to react with oxygen to form Aluminium Oxide (##Al_2O_3##), the oxidation number for aluminium would be ##+3## since oxygen is ##-2## (rule 5) and there are 3 oxygen atoms in a molecule so each aluminium atom must be ##+3## charge to satisfy rule 3.