Rules for assigning oxidation numbers

Return to Redox Menu


I. Rule Number One

All free, uncombined elements have an oxidation number of zero.

This includes diatomic elements such as O2 or others like P4 and S8.

II. Rule Number Two

Hydrogen, in all its compounds except hydrides, has an oxidation number of +1 (positive one)

III. Rule Number Three

Oxygen, in all its compounds except peroxides, has an oxidation number of -2 (negative two).

With only a very few exceptions, oxidation states can be assigned to all atoms in a formula. There are more extensive sets of rules and, for the most part, they derive from the three above rules.

There are some complex examples which are not discussed in the usual set of introductory rules. For example, potassium superoxide is KO2. That means that the oxidation number on the O2 is negative one. These more complex examples will not be taught in this tutorial.


Now, some examples:

1. What is the oxidation number of Cl in HCl?

Since H = +1, the Cl must be -1 (minus one).

2. What is the oxidation number of Na in Na2O?

Since O = -2, the two Na must each be +1.

3. What is the oxidation number of Cl in ClO¯?

The O is -2, but since a -1 must be left over, then the Cl is +1.

4. What is the oxidation number for each element in KMnO4?

K = +1 because KCl exists. We know the Cl = -1 because HCl exists. O = -2 by definition Mn = +7. There are 4 oxygens for a total of -8, K is +1, so Mn must be the rest.

5. What is the oxidation number of S in SO42¯

O = -2. There are four oxygens for -8 total. Since -2 must be left over, the S must = +6.

Please note that, if there is no charge indicated on a formula, the total charge is taken to be zero.


Practice Problems

What is the oxidation number of . . .

1) N in NO3¯

2) C in CO32¯

3) Cr in CrO42¯

4) Cr in Cr2O72¯

5) Fe in Fe2O3

6) Pb in PbOH+

7) V in VO2+

8) V in VO2+

9) Mn in MnO4¯

10) Mn in MnO42¯


Go to Answers

Return to Redox Menu