← Chemistry F4–F6

HKDSE CHEMISTRY

Oxidation Number Master & MC Assessment

HKDSE Core Skill - Page 1

HKDSE Oxidation State Rules

Oxidation numbers represent the apparent charge of an atom in a compound or ion based on electronegativity rules. Master these fundamental rules to solve any HKDSE redox question with 100% accuracy!

Rule Priority Hierarchy

Strict HKDSE Priority Order
1 Uncombined Elements

Any element in its free uncombined elemental state has an oxidation number of 0.

Examples: Cl2 = 0, O2 = 0, Na = 0, Fe = 0, S8 = 0
2 Monatomic Ions

The oxidation state of a monatomic ion is equal to its actual ionic charge.

Examples: Fe3+ = +3, Na+ = +1, I- = -1, S2- = -2
3 Hydrogen Rules

Hydrogen is almost always +1 in compounds.

Exception: In metal hydrides (e.g. NaH, CaH2), H = -1.
4 Oxygen Rules

Oxygen is usually -2 in compounds.

Exceptions: Peroxides (H2O2) O = -1; OF2 O = +2.
5 Group Metals & Fluorine

Group I metals = +1, Group II metals = +2, Fluorine = -1 always.

Examples: K in KMnO4 = +1, Ca in CaCO3 = +2, F = -1
6 Complex Ion Ligands

Neutral ligands (NH3, H2O) = 0. Anionic ligands (CN-, Cl-) equal their ionic charge.

Example: In [Fe(CN)6]3-, CN- = -1, so Fe + 6(-1) = -3 ⇒ Fe = +3

The Sum Rule Equation

In any neutral compound, the sum of oxidation numbers of all atoms is 0. In a polyatomic ion or complex ion, the sum equals the overall charge of the ion.

Neutral Compound Example: KMnO4: (+1) + Mn + 4(-2) = 0 ⇒ Mn - 7 = 0 ⇒ Mn = +7
Polyatomic Ion Example: Cr2O72-: 2(Cr) + 7(-2) = -2 ⇒ 2Cr = +12 ⇒ Cr = +6