# Oxidation states of oxygen

4. Oxygen in a compound is -2, unless a peroxide is present. The oxidation state of oxygen in peroxide ion, O22- is -1.
5. For compounds containing polyatomic ions, use the overall charge of the polyatomic ion to determine the charge of the cation.

Here is a convenient method for determining oxidation states. Basically, you treat the charges in the compound as a simple algebraic expression. For example, let's determine the oxidation states of the elements in the compound, KMnO4. Applying rule 2, we know that the oxidation state of potassium is +1. We will assign "x" to Mn for now, since manganese may be of several oxidation states. There are 4 oxygens at -2 apiece. The overall charge of the compound is zero:

K Mn O4
+1 x 4(-2)

The algebraic expression generated is:

1 + x -8 = 0

Solving for x gives the oxidation state of manganese:

x - 7 = 0

x = +7

K Mn O4
+1 +7 4(-2)

Suppose the species under consideration is a polyatomic ion. For example, what is the oxidation state of chromium in dichromate ion, (Cr2O72-)? As before, assign the oxidation state for oxygen, which is known to be -2. Since the oxidation state for chromium is not known, and two chromium atoms are present, assign the algebraic value of 2x for chromium:
Cr2 O7 2-
2x 7(-2)
Set up the algebraic equation to solve for x. Since the overall charge of the ion is -2, the expression is set equal to -2 rather than 0:
2x + 7(-2) = -2
Solve for x:
2x - 14 = -2

2x = 12

x = +6

Each chromium in the ion has an oxidation state of +6.
Let's do one last example, where a polyatomic ion is involved. Suppose you need to find the oxidation state of all atoms in Fe2(CO3)3. Here two atoms, iron and carbon, have more than one possible oxidation state. What happens if you don't know the oxidation state of carbon in carbonate ion? In fact, knowledge of the oxidation state of carbon is uneccessary. What you need to know is the charge of carbonate ion (-2). Set up an algebraic expression while considering just the iron ion and the carbonate ion:
Fe2 (CO3)3
2x 3(-2)

2x - 6 = 0

2x = 6

x = 3

Each iron ion in the compound has an oxidation state of +3.
Next consider the carbonate ion independent of the iron(III) ion:
C O3 2-
x 3(-2)

x - 6 = -2

x = +4

The oxidation state of carbon is +4 and each oxygen is -2.