Assigning Oxidation Numbers

One of the easiest tools designed by the chemistry to track the electrons gained or lost during a redox reaction are the oxidation numbers
Each atom in a neutral compound or ion is assigned with an oxidation number
Oxidation numbers are the “charges” that atoms will have, if the shared electrons in a bond were assigned to the atom with the greatest attraction for those electrons

Rules for assigning oxidation numbers

The oxidation number of an atom that is a free element is 0
- E.g. Copper (Cu) has 0
Diatomic elements have also an oxidation number of 0
- E.g. Chlorine gas (Cl₂) has 0
In ions of just one atom, the oxidation number is equal to its charge
- E.g. Ca²⁺ has +2 or Cl^- has -1
The sum of the oxidations numbers of all the atoms in a neutral compound is 0
- E.g. H₂O has 2 hydrogen atoms with +1 and 1 oxygen atom with -2
- 2(H oxidation number) + 1(O oxidation number) = 0
- 2(+1) + 1(-2) = 0
- 0 = 0
The sum of the oxidation numbers of all the atoms in a polyatomic ion is equal to the charge of the ion
- E.g. NO₃^- has 1 nitrogen atom with +5 and 3 oxygen atom with -2
- 1(N oxidation number) + 3(O oxidation number) = charge of the ion
- 1(+5) + 3(-2) = -1
- -1 = -1
Metals have always positive oxidation number in compounds
- Group 1 metals have always +1
- Group 2 metals have always +2
Non metals can acquire positive and negative oxidation numbers depending on the compound they are forming
- In most scenarios, hydrogen tends to have +1 oxidation number
- The negative oxidation number of nonmetals from Group 15 to 16 can be calculated using the formula:
  negative oxidation number = valence electrons - 8
- The table below show common oxidations numbers for non metals

Table showing the negative oxidation numbers for nonmetals from Group 15 to 17

Nonmetal	Valence electrons	Oxidation number
Group 17	7	-1
Group 16	6	-2
Group 15	5	-3

8. The oxidation number of any element can be figured out using rules 4 and 5, once all other oxidation numbers have been assigned

9.In chemical formulas, element with positive oxidation numbers tend to be on the left side of the formula, while elements with negative oxidation numbers tend to be on the right side

Exam Tip

The sign of the charge in oxidation numbers is written before the numeric quantity in ions. E.g. The oxidation number of Ca²⁺ is +2. The oxidation number is not the same as the ionic charge

Worked example

Assign the oxidation number of all the atoms in the following compounds or ions:

CO₂
SO₄^2-

Answer a:

Using rule 9, elements with negative oxidation numbers tend to be on the right side of the formula. Therefore O oxidation number = -2
Using rule 4, the sum of all the oxidations numbers of the elements in a neutral molecule must be 0

1(C oxidation number) + 2(O oxidation number) = 0

1(C oxidation number) + 2(-2) = 0

1(C oxidation number) - 4 = 0

1(C oxidation number) = 4

C oxidation number = +4

Answer b:

Using rule 9, elements with negative oxidation numbers tend to be on the right side of the formula. Therefore O oxidation number = -2
Using rule 4, the sum of all the oxidation numbers of the elements in a polyatomic ion must be equal to its charge. Sulfate has a charge of 2- so the sum must be equal to -2

1(S oxidation number) + 4(O oxidation number) = -2

1(S oxidation number) + 4(-2) = -2

1(S oxidation number) - 8 = -2

1(S oxidation number) = -2 + 8

S oxidation number = +6

Assigning Oxidation Numbers (College Board AP Chemistry)

Revision Note