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– |
| 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.
- P2O5
- SO42–
- H2S
- Al2Cl6
- NH3
- ClO2–
- CaCO3
Answer
- 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
- SO42–
- 4 O atoms = 4 x (–2) = –8
- The overall charge of the compound = –2
- The oxidation number of 1 S atom = +6
- H2S
- 2 H atoms = 2 x (+1) = +2
- The overall charge of the compound = 0
- The oxidation number of 1 S atom = –2
- 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
- NH3
- 3 H atoms = 3 x (+1) = +3
- The overall charge of the compound = 0
- The oxidation number of 1 N atom = –3
- ClO2–
- 2 O atoms = 2 x (–2) = –4
- The overall charge of the compound = –1
- The oxidation number of 1 Cl atom = +3
- 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
