Cracking the SAT Chemistry Subject Test

Oxidation States and Oxidation Numbers

Each atom in a compound can also be assigned what’s called an oxidation state.
The oxidation state is positive if the atom is likely to lose electrons and negative
if the atom is likely to gain electrons.

If, for instance, we imagine that some atom has gained two electrons, its
oxidation state is −2, and if it has lost one electron, its oxidation state would be
+1. Since total reduction has to equal total oxidation for all compounds, the sum
of alloxidation numbers of a compound is always zero. It is important to
remember that we assign oxidation states to atoms only when they aren’t in their
elemental forms. For example, each atom of H 2 , O 2 , Cl 2 , N 2 , Na, or Fe has an

oxidation state of zero. Atoms in all other compounds, such as those in H 2 CO 3 ,

CaO, or N 2 O, can be assigned an oxidation number that is not zero.

On the SAT Chemistry Subject Test, the writers might show you a compound
and ask you to calculate the oxidation states of its atoms. If you remember a few
simple rules, you’ll always be able to answer these questions.

For some elements, the oxidation state is almost always the same, no matter
what compound they’re sitting in, while for other elements, the oxidation state
varies depending on the compound. Here are the important points to remember.

1. 
   

        When    oxygen  is  in  a   compound,   its oxidation   state   is  usually −2  (it has

   
   

   been reduced). One important exception is oxygen in a peroxide such as
   hydrogen peroxide (H 2 O 2 ). In a peroxide, oxygen has an oxidation state of
   −1.

   
   


2. When an alkali metal (Li, Na, etc.) is involved in a compound, its
   oxidation state is always +1 (it’s been oxidized).


3. When an alkaline earth metal (Be, Mg, etc.) is involved in a compound,
   its oxidation state is +2.


4. When a halogen (F, Cl, etc.) is involved in a compound, its oxidation
   state is often −1. The oxidation state of fluorine in a compound is always
   −1.


5. When hydrogen is combined with a nonmetal, its oxidation state is +1.
   When hydrogen is combined with a metal, its oxidation state is −1.


6. In any compound, the sum of all oxidation states is zero.