1.9.2Physical Chemistry
Oxidation States: The Rules — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 51 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5The Core Rules
Oxidation state is the charge an atom would have if all its bonds were purely ionic.
Uncombined elements
- Uncombined elements Any element on its own, such as Cu, O₂ or P₄, has oxidation state zero.
Simple ions
- Simple ions The oxidation state of a monatomic ion equals its charge, so Fe³⁺ is +3 and Cl⁻ is −1.
Sum rule
- Sum rule The oxidation states in a neutral compound add up to zero, and in an ion they add up to the ion charge.
Group 1 and 2
- Group 1 and 2 Group 1 metals are always +1 in compounds and Group 2 metals are always +2.
Fluorine
- Fluorine Fluorine is always −1 in compounds because it is the most electronegative element.
Part 2 of 5Elements With Variable States
Elements With Variable States
Oxygen
- Oxygen Normally −2, but in peroxides each oxygen is −1 because of the O–O bond.
Hydrogen
- Hydrogen Normally +1 when bonded to non-metals, but −1 when bonded to a metal as a hydride.
Halogens
- Halogens Chlorine, bromine and iodine are −1 in halides but positive when bonded to more electronegative oxygen.
| Element | Usual state | Exceptions |
|---|---|---|
| Oxygen | −2 | −1 in peroxides such as H₂O₂; +2 in OF₂ |
| Hydrogen | +1 | −1 in metal hydrides such as NaH |
| Chlorine | −1 | Positive when combined with oxygen or fluorine |
Part 3 of 5Naming and Using Oxidation States
1
Roman numerals in names show the oxidation state of the metal, so iron(III) oxide contains Fe³⁺ and manganate(VII) contains Mn in the +7 state.
2
Write down known oxidation states first, then use the sum rule to work out the unknown element algebraically.
3
Compare the oxidation state of an element before and after reaction to decide whether it has been oxidised or reduced.
4
Oxidation states are written with the sign first, as +3 or −2, while ionic charges are written with the number first, as 3+ or 2−.
Part 4 of 5Worked Example
Question Find the oxidation state of (a) Mn in KMnO₄ and (b) Cr in Cr₂O₇²⁻.
- (a) K is +1 and each O is −2: +1 + x + 4(−2) = 0, so x = +7.
- (b) Each O is −2, so 7 O atoms give −14; the two Cr atoms must give +12 to reach an overall charge of −2.
- Equation2x + 7(−2) = −2, so 2x = +12 and x = +6 for each chromium.
Answer Mn is +7 in KMnO₄ and Cr is +6 in Cr₂O₇²⁻.
Part 5 of 5Exam Checklist & Tips
1.9.2 – Key Points for the Exam
- Oxidation state The charge an atom would have if all its bonds were completely ionic.
- Peroxide A compound containing the O–O bond, in which oxygen has oxidation state −1.
- Hydride A compound of hydrogen with a metal, in which hydrogen has oxidation state −1.
- Sum rule Oxidation states add up to the overall charge on the species.
Common Mistakes
- Assuming oxygen is always −2, which fails in peroxides and OF₂.
- Writing the sign after the number, or confusing oxidation state with ionic charge notation.
- Forgetting that the oxidation state of an element in its elemental form is zero.
- Giving the oxidation state per molecule instead of per atom, for example Cr in Cr₂O₇²⁻.
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