- Oxidation numbers are arbitrary numbers assigned to atoms to describe their relative state of oxidation or reduction i.e. how oxidised or reduced they are.
- They are assigned according to an arbitrary set of rules:
|
Species |
Oxidation No. (ON) |
Example: Species (ON) |
|
Free Elements |
= Zero |
Mg (0), Cl2 (0), S8 (0) |
|
Monoatomic Ions |
= Charge on Ion |
Ca2+ (+2), O2– (-2) |
|
Polyatomic Ions |
Sum of ON = Charge on Ion * |
SO42– Let the ON of S be x. x + 4(-2) = -2; x = +6 |
|
Neutral Compounds/ Molecules |
Sum of ON = Zero * |
NCl3 Let the ON of N be x. x + 3(-1) = 0; x = +3 |
* requires memorising of the common ON of elements:
|
Element |
Oxidation No. (ON) |
|
Hydrogen (H) |
+1 |
|
Oxygen (O) |
-2 |
|
Halogens (X) |
-1 |
Practice:
Determine the oxidation numbers of the underlined atoms in the following species:
(a) P4
(b) Cr3+
(c) VO2+
(d) MnO2
Universal method for determining ON:
- Break all bonds in species
- Assign shared e- to more electro-ve atom
- ON = resultant charge on atom
Example: HCN
Practice:
Determine the oxidation numbers of the elements in carbon disulfide CS2.
