**I. Rule Number One**

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

This includes the seven diatomic elements (such as O

_{2}) and other molecular elements (P_{4}and S_{8}).

**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 and superoxides, has an oxidation number of -2 (negative two).

**IV. Rule Number Four**

When no charge is indicated in a given formula, the total charge is taken to be zero.

With only a very few exceptions (which you will never see at an introductory level), 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 four rules above.

**Example #1:** What is the oxidation number of Cl in HCl?

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

**Example #2:** What is the oxidation number of Na in Na_{2}O?

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

**Example #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.

**Example #4:** What is the oxidation number of Cl in ClO_{2}¯?

Two O is -4 (from -2 x 2), but since a -1 must be left over, then the Cl is +3.

**Example #5:** What is the oxidation number of Cl in ClO_{3}¯?

Three O is -6 (from -2 x 3), but since a -1 must be left over, then the Cl is +5.

**Example #6:** What is the oxidation number of Cl in ClO_{4}¯?

Four O is -8 (from -2 x 4), but since a -1 must be left over, then the Cl is +7.

**Example #7:** What is the oxidation number of S in SO_{4}^{2}¯

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

**Example #8:** What is the oxidation number of S in SO_{3}^{2}¯

O = -2. There are three oxygens for -6 total. Since -2 must be left over, the S must = +4.

**Example #9:** What are the oxidation numbers in KCl?

K = +1 because K_{2}O exists. The O is -2 by defintion, therefore each K must be +1 in order to keep the K_{2}O formula at zero charge.Cl = -1 because HCl exists. H is a +1 by definition, therefore Cl must be a -1. You can also say that Cl is a -1 because K must be a +1 and we need to have a zero charge for the formula.

**Example #10:** What is the oxidation number for each element in NaMnO_{4}?

Na = +1 because Na_{2}O exists. We know the O = -2, so each Na is a +1. (We could also use this: we know NaCl exists and that Cl is a -1. We know this because HCl exists. Therefore, the Na is a +1.)O = -2 by definition

Mn = +7. There are 4 oxygens for a total of -8, K is +1, so Mn must be the rest.

**Practice Problems**

What is the oxidation number of . . .

1) N in NO_{3}¯

2) C in CO_{3}^{2}¯

3) Cr in CrO_{4}^{2}¯

4) Cr in Cr_{2}O_{7}^{2}¯

5) Fe in Fe_{2}O_{3}

6) Pb in PbOH^{+}

7) V in VO_{2}^{+}

8) V in VO^{2+}

9) Mn in MnO_{4}¯

10) Mn in MnO_{4}^{2}¯