Question

How do you calculate the oxidation number of an ion?

Answer 1

For simple ions, i.e. `Na^+`, `Cl^-`, `Fe^(2+)`, `Fe^(3+)`, the oxidation number of the element is simply the charge on the ion.

Explanation:

For more complex ions, the SUM of the oxidation numbers equals the charge on the ion. The sum of the oxidation numbers also equals the charge on the molecule in a NEUTRAL molecule.

Let's consider `Cl^-`, `ClO_2`, `ClO_4^-`. Typically the oxidation number of oxygen in its oxides is `-II`, and it is here in these examples.

Given what I have said the oxidation number of `Cl` in chloride ion, `Cl^-`, is `-I`. The oxidation number of `Cl` in `ClO_2` is `+IV`, and the oxidation number of `Cl` in perchlorate, `ClO_4^-`, is `+VII`.

I don't what level you are at. If you are at A level, you simply have to know what I wrote in the opening statement. If you are an undergrad you do have to know how to assign oxidation states in more complicated molecules and ions. Good luck.