Chemists have devised a useful “accountancy” tool to help keep track of electrons in compounds and reactions. This is particularly important in
redox reactions where some atoms
lose (are
oxidised) and others
gain (are
reduced) electrons.
Lewis structures are another "accountancy" tool for molecules - Lewis structures and oxidation numbers are intimately linked.
Each atom in a molecule is assigned an oxidation number (sometimes called oxidation state). This is the positive or negative charge the atom would have if the molecule was ionic.
These rules should be used in the order that they are written:
- The sum of the oxidation numbers of the atoms in a molecule add up to zero and in an ion add up to the charge of the ion.
- The oxidation number of fluorine is -1 (except in F2 where it is 0 [rule 1]).
- The oxidation number of group 1 elements is +1.
- The oxifation number of group 2 elements is +2.
- The oxidation number of oxygen is -2, except in peroxides where it is -1 and in O2 where it is 0 [rule 1] and in OF2 where it is +2 [rule 2].
- The oxidation number of halogens is usually -1 [except where rule 1 or 2 is important].
- The oxidation number of hydrogen is +1 when bonded to non-metals and -1 when bonded to metals.
AlF3:
F has an oxidation number of -1 [rule 2]. Al must have oxidation number of +3 so that the overall charge is zero [rule 1].
AlF3 ≡ Al3+ + 3F- as (+3) + (3 × -1) = 0.
NH3:
H is attached to a non-metal so oxidation number of +1 [rule 7]. N must have oxidation number of -3 so that the overall charge is zero [rule 1].
NH3 ≡ N3- + 3H+ as (-3) + (3 × +1) = 0.
NH4+:
H is attached to a non-metal so oxidation number of +1 [rule 7]. N must have oxidation number of -3 so that the overall charge of the ion is +1 [rule 1].
NH4+ ≡ N3- + 4H+ as (-3) + (4 × +1) = +1.
NH2-:
H is attached to a non-metal so oxidation number of +1 [rule 7]. N must have oxidation number of -3 so that the overall charge of the ion is -1 [rule 1].
NH2- ≡ N3- + 2H+ as (-3) + (2 × +1) = -1.
Cl2:
The overall charge of the molecule is 0 so the oxidation number of each Cl must also be 0 [rule 1].
Cl2 ≡ Cl0 as (2 × 0) = 0.
ClF3:
Fluorine has an oxidation number of -1 [rule 2]. Cl must have an oxidation number +3 so that the overall charge is zero [rule 1].
ClF3 ≡ Cl3+ + 3F- as (+3) + (3 × -1) = 0.
ClF4-:
Fluorine has an oxidation number of -1 [rule 2]. Cl must have an oxidation number +3 so that the overall charge of the ion is -1 [rule 1].
ClF4- ≡ Cl3+ + 4F- as (+3) + (4 × -1) = -1.
ClF4+:
Fluorine has an oxidation number of -1 [rule 2]. Cl must have an oxidation number +5 so that the overall charge of the ion is +1 [rule 1].
ClF4+ ≡ Cl5+ + 4F- as (+5) + (4 × -1) = +1.
These quizzes are randomly generated so you can take the quiz multiple times to test your knowledge and understanding.
