Oxidation States

Skills to develop

Assign an oxidation state (or oxidation number) to an atom in a molecule.

Oxidation State

Oxidation state is a number assigned to an element in a compound according to some rules. This number enable us to describe oxidation-reduction reactions. For the purpose of balancing oxidation-reduction reactions, You have to assign oxidation states (or oxidation numbers) to a variety of compounds or ions.

When an oxidation number is assigned to the element, it does not imply that the element in the compound acquires this as a charge, but rather that it is a convenient number to use for balancing chemical reactions. The guidelines for assigning oxidation states (numbers) are given below:

The oxidation state of any element such as Fe, H₂, O₂, P₄ and S₈, is zero (0).
The oxidation state of oxygen in its compounds is -2, except for peroxides like H₂O₂, and Na₂O₂, in which the oxidation state for O is -1.
The oxidation state of hydrogen is +1 in its compounds, except for metal hydrides, such as NaH, LiH, LiAlH₄, etc., in which the oxidation state for H is -1.
The oxidation states of other elements are then assigned to make the algebraic sum of the oxidation states equal to the net charge on the molecule or ion.

The following elements usually have the same oxidation states in their compounds:

+1 for alkali metals - Li, Na, K, Rb, Cs;
+2 for alkaline earth metals - Be, Mg, Ca, Sr, Ba;
-1 for halogens except when they form compounds with oxygen or one another;

Examples of oxidation state assignment

Element	Oxidation state	Compound or ion
H	+1	H⁺
	+1	H₂O

Cl	-1	Cl^-
	0	Cl₂
	+1	ClO^-
	+3	ClO₂^-
	+4	ClO₂
	+5	ClO₃^-
	+7	ClO₄^-

N	-3	NH₃
	-2	N₂H₄
	-1	NH₂OH
	0	N₂
	+1	N₂O
	+2	NO
	+3	NO₂^-
	+4	NO₂
	+5	NO₃^-

In the examples given in the table here, take a look at the compound or ion, and assign the oxidation state (or oxidation number) to all elements. The oxidation state of one element is given here as a guidance.

The following are some common oxidants or reductants. Changes of oxidation states in redox reactions of the key elements are given in groups. You should justify the assigned oxidation state to your satisfaction as you read on.

Element Oxidation
state Compound
or ion
Fe +2 Fe²⁺
+3 Fe³⁺

Zn 0 Zn
+2 Zn²⁺

O -1 H₂O₂
0 O₂
-2 H₂O

Cr +6 Cr₂O₇^2-
+3 Cr³⁺
+6 CrO₄^2-

Mn +7 MnO₄^-
+6 MnO₄^2-
+4 MnO₂
+2 Mn²⁺

C +3 H₂C₂O₄
+4 CO₂
+4 CO₃^2-
+2 CO

Element	Oxidation state	Compound or ion
Fe	+2	Fe²⁺
	+3	Fe³⁺

Zn	0	Zn
	+2	Zn²⁺

O	-1	H₂O₂
	0	O₂
	-2	H₂O

Cr	+6	Cr₂O₇^2-
	+3	Cr³⁺
	+6	CrO₄^2-

Mn	+7	MnO₄^-
	+6	MnO₄^2-
	+4	MnO₂
	+2	Mn²⁺

C	+3	H₂C₂O₄
	+4	CO₂
	+4	CO₃^2-
	+2	CO

Confidence Building Questions

What is the oxidation state of Mo in MoO₄^2-?

Skill:
Note that (+6) + 4*(-2) = -2, the charge of the ion MoO₄^2-. An oxidation state is a signed number, in this case +6.
What is the oxidation state of S in SO₃^2-?

Discussion:
Even though S is in group VIA: O, S, Se, Te, Po, the sulfur in sulfite ions has an oxidation number +4.
What is the oxidation state of As in AsO₄^3-?

Discussion:
This is the normal oxidation state of group VA: N, P, As, Sb, Bi.
What is the oxidation state of Br in BrO₃^-?

Discussion:
Br has many oxidation states, e.g. +7 in BrO₄^-.
What is the oxidation state of As in As₂S₃?

Discussion:
The oxidation for S is -2, since it is in the group VIA: O, S, Se, Te, Po. Super! S being in the same group as O, has an oxidation state of -2.
What is the oxidation state of Cr in Cr(OH)₃?

Discussion:
There are three OH^- groups in the compound. Chromium hydroxide, Cr(OH)₃, is a greenish gelatinous solid.