Basic Oxidation Numbers
Cambridge IGCSE Chemistry 0620 · Redox & Electron Transfer · Supplement
The Golden Rules
0 of 15 answers revealed
Section A
Elements — Pure & Unchained
State the oxidation number of sulfur in S₈.
💡 Sulfur exists as pure element molecules.
State the oxidation number of hydrogen in H₂ gas.
💡 Think: is H₂ a compound or a pure element?
State the oxidation number of oxygen in O₂.
💡 Pure oxygen gas — same rule applies.
State the oxidation number of chlorine in Cl₂ gas.
💡 Diatomic halogen — pure element.
State the oxidation number of iron in Fe(s).
💡 Solid iron metal — pure element.
Section B
Monatomic Ions — Reading the Charge
State the oxidation number of the bromide ion, Br⁻.
💡 For monatomic ions, oxidation number = ionic charge.
State the oxidation number of aluminium in Al³⁺.
💡 Al³⁺ has lost 3 electrons — what does that make its oxidation number?
State the oxidation number of zinc in Zn²⁺.
💡 Zinc always forms the same ion in all its compounds.
State the oxidation number of the oxide ion, O²⁻.
💡 Oxygen gained 2 electrons to form this ion.
State the oxidation number of copper in Cu²⁺.
💡 Read the charge directly from the ion symbol.
Section C
Comparisons — Tracking the Change
How does the oxidation number of copper change when Cu turns into Cu²⁺?
💡 Start with pure metal, end with an ion.
How does the oxidation number of iron change when Fe³⁺ turns into Fe²⁺?
💡 Is the oxidation number going up or down?
Magnesium burns in oxygen: 2Mg + O₂ → 2MgO. What is the oxidation number of magnesium in MgO?
💡 Mg always forms Mg²⁺. What must oxygen be to balance?
Chlorine gas is bubbled into sodium iodide solution: Cl₂ + 2NaI → 2NaCl + I₂. What is the change in oxidation number of chlorine?
💡 Cl₂ is a pure element. NaCl contains Cl⁻.
In the reaction: Zn + CuSO₄ → ZnSO₄ + Cu, what are the oxidation number changes for BOTH zinc and copper?
💡 This is a full displacement reaction — identify what is oxidised and what is reduced.
🎓 Examiner's Final Briefing
Rule 1: Pure element → always 0. No exceptions. H₂, O₂, Cl₂, S₈, Fe(s), Cu(s) — all zero.
Rule 2: Monatomic ion → oxidation number = the charge. Al³⁺ = +3. Br⁻ = −1. Write the sign.
Rule 3: Oxidation = INCREASE in ox. no. (loss of e⁻). Reduction = DECREASE (gain of e⁻). Use OIL RIG.
Rule 4: Cambridge demands the + sign for positive oxidation numbers. '3' ≠ '+3'. Lose the mark.
Sign In to Keep Going
You've reached the end of the free preview. Sign in — and upgrade to Plus or Pro — to keep going with Chemistry.
Full Access
Unlock all 320 lessons