Cracking the SAT Chemistry Subject Test

Oxidation   States

Oxygen: −2

Alkali  Metals: +1

Alkaline    Earth   Metals: +2

Halogens:   −1

Hydrogen:   ±1

Remember those six simple rules, and you’ll be able to answer the questions
about oxidation state on the exam. Here’s an example.

What     is  the     oxidation   state   for     nitrogen    in  the     compound    nitrogen    monoxide

(NO)?

Oxygen’s oxidation number is −2; there’s 1 oxygen atom in the molecule, so
oxygen contributes a total reduction of −2.

Since the total reduction (gain of electrons) must equal the total oxidation (loss
of electrons), nitrogen must have an oxidation state of +2 (which means it has
been oxidized, having lost two electrons).

Try another one.

What    is  the oxidation   number  of  carbon  in  iron(III)   carbonate,  Fe 2 (CO 3 ) 3?

If Fe’s subscript is 2 and CO 3 ’s is 3, that means the charge on Fe must be +3, and

the charge on CO 3 must be −2. The oxidation state of an ion is equal to its

charge. So the oxidation state of Fe is +3. The total oxidation state of CO 3 is −2.

The oxidation state of oxygen is −2. Since there are 3 oxygen atoms in CO 3 , the

total contribution by oxygen is −6. This means that carbon’s oxidation state must
be +4 so that CO 3 ’s total oxidation state is (+4) + 3(−2) = −2.

Now let’s consider how these oxidation states add to give the total oxidation
state of Fe 2 (CO 3 ) 3.