OXIDATION NUMBERS

1. Oxidation numbers and oxidation states

The OXIDATION NUMBERS, frequently also called the OXIDATION STATES () of atoms is a concept which enable us to

Let us take the case of a simple molecule, hydrogen chloride, HCl. Of the two atoms, chlorine is the most electronegative, and in deciding the oxidation numbers of the two atoms, the pair of electrons that forms the covalent bond are IMAGINED to have been transferred to that chlorine atom. This results in the chlorine atom having one additional electron, while the hydrogen atom has lost its one and only electron:

On this basis we assign an oxidation number of +1 to the hydrogen atom, and -1 to the chlorine atom.

For many ionic compounds, it is easy to determine the oxidation number of the atoms. For example, iron forms two chlorides, FeCl2 and FeCl3. The chlorine ions have an oxidation number of -1, so in FeCl2, iron has an oxidation number of +2 while in FeCl3, iron has an oxidation number of +3. We refer to these salts as iron(II) chloride and iron(III) chloride respectively, the Roman numerals indicating the oxidation state of the iron atom in each case.

It is very important to remember the following:

OXIDATION results in an INCREASE in oxidation number.
REDUCTION results in a DECREASE in oxidation number.

2. Rules for determining oxidation numbers

Oxidation numbers can be readily determined by the application of simple rules:

  1. The oxidation number of a free, uncombined element is zero.
  2. For a neutral compound, the sum of the oxidation numbers of all the atoms in the compound's formula is zero.
  3. For an ion, the sum of the oxidation numbers of all the atoms in the formula for the ion is equal to the charge on the ion. This means also that the oxidation number of a monoatomic ion is simply the charge on that ion.
  4. The oxidation number for hydrogen, in combination with other atoms, is always equal to +1 (except for hydrides, H-, which must be -1 according to rule 3 above).
  5. The oxidation number for oxygen, in combination with other atoms, is always equal to -2 (except for peroxides, with the anion O22-, where oxygen has an oxidation number -1)

3. Applying the oxidation number rules

3.1 Potassium permanganate, KMnO4

Potassium permanganate is an ionic compound made up of two ions, K+ (rule 3 gives the oxidation number +1) and the permanganate ion MnO4- (rule 3 gives the oxidation number -1). The sum of the oxidation numbers of all the atoms in that ion must equal -1, but we know from rule 2 that oxygen contributes 4x(-2) = -8 to that value, meaning that Mn has an oxidation number of +7.

3.2 Sodium dichromate, Na2Cr2O7

Sodium dichromate is an ionic compound made up of three ions, 2 Na+ (rule 3 gives the oxidation number of sodium ions as +1) and the dichromate ion Cr2O72- (rule 3 gives the oxidation number -2). The sum of the oxidation numbers of all the atoms in that ion must equal -2, but we know from rule 2 that oxygen contributes 7x(-2) = -14 to that value, meaning that the two Cr atoms have a total oxidation number of +12, in other words, each Cr atom has an oxidation number of +6.

3.3 Sodium borohydride NaBH4

Sodium borohydride is an ionic compound made up of 2 ions, Na+ (rule 3 gives the oxidation number of sodium ions as +1) and the borohydride ion BH4-. The sum of the oxidation numbers in that ion must be -1, but we know that hydrogen in hydrides has an oxidation number of -1, and therefore hydrogen contribute -4 to the oxidation number of the ion. This means that boron has an oxidation number of +3.


4. Additional questions