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ASChemistryCambridge (CIE)Revision Notes6. ElectrochemistryRedox Processes: Electron Transfer & Changes in Oxidation Number (Oxidation State)Oxidation Number Rules

Oxidation Number Rules (Cambridge (CIE) AS Chemistry)

Revision Note

Caroline Carroll

Last updated

Oxidation Numbers

  • The oxidation number (also known as oxidation state) is a number given to each atom or ion in a compound to keep track of how many electrons they have

  • In a single ion or molecular ion, the oxidation number tells us how many electrons have been lost or gained

  • Positive oxidation number = loss of electrons

  • Negative oxidation number = gain of electrons

Oxidation number rules

  • The oxidation number refers to a single atom in a compound

 

Rule

Example

1

The oxidation number of any uncombined element is zero

H2 
Zn
O2 

2

Many atoms or ions have fixed oxidation numbers in compounds

Group 1 elements are always +1
Group 2 elements are always +2 Fluorine is always –1
Hydrogen is +1, except in hydrides like NaH where it is –1 Oxygen is –2, except in peroxides where it is in –1 and in F2O where it is +2

3

The oxidation number of an element in a monoatomic ion is always the same as the charge

Zn2+ = +2
Fe3+ = +3
Cl– = –1

4

The sum of the oxidation numbers in a compound is zero

NaCl
Na = +1
Cl = –1
Sum of oxidation numbers = 1 – 1 = 0

5

 The sum of the oxidation numbers in an ion is equal to the charge on the ion

SO42–
S = +6
Four O atoms = 4 x (–2) = –8
Sum of oxidation numbers = 6 – 8 = –2

6

In either a compound or an ion, the more electronegative element is given the negative oxidation number

F2O
Two F atoms = 2 x (–1) = –2
O = +2

Worked Example

Deducing oxidation numbers

State the oxidation number of the bold atoms in these compounds or ions.

  1. P2O5

  2. SO42– 

  3. H2S

  4. Al2Cl6

  5. NH3 

  6. ClO2– 

  7. CaCO3 

Answer

  1. P2O5 

    • 5 O atoms = 5 x (–2) = –10

    • The overall charge of the compound = 0

    • 2 P atoms = +10

    • Oxidation number of 1 P atom = (+10) / 2 = +5

  2. SO42– 

    • 4 O atoms = 4 x (–2) = –8

    • The overall charge of the compound = –2

    • The oxidation number of 1 S atom = +6

  3. H2S

    • 2 H atoms = 2 x (+1) = +2

    • The overall charge of the compound = 0

    • The oxidation number of 1 S atom = –2

  4. Al2Cl6 

    • 6 Cl atoms = 6 x (–1) = –6

    • The overall charge of the compound = 0

    • 2 Al atoms = +6

    • The oxidation number of 1 Al atom = (+6) / 2 = +3

  5. NH3 

    • 3 H atoms = 3 x (+1) = +3

    • The overall charge of the compound = 0

    • The oxidation number of 1 N atom = –3

  6. ClO2– 

    • 2 O atoms = 2 x (–2) = –4

    • The overall charge of the compound = –1

    • The oxidation number of 1 Cl atom = +3

  7. CaCO3 

    • 3 O atoms = 3 x (–2) = –6

    • 1 Ca atom = +2

    • The overall charge of the compound = 0

    • The oxidation number of 1 C atom = +4

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    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.