Oxidation states and identifying redox
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Question
What is an oxidation state?
Answer
A number tracking how many electrons an atom has **gained or lost** relative to the free element — the charge it would have if all bonds were ionic.
Question
Oxidation state of a free, uncombined element?
Answer
Always **0** (e.g. Na, O_{2}, Cl_{2}, S_{8}).
Question
Oxidation state of a simple monatomic ion?
Answer
**Equal to its charge** (e.g. Mg²⁺ is +2, Cl⁻ is −1).
Question
Usual oxidation state of oxygen? Of hydrogen?
Answer
Oxygen is **−2**; hydrogen is **+1** — except peroxides (O is −1) and metal hydrides (H is −1).
Question
How do oxidation states sum in a species?
Answer
They add up to the **total charge**: 0 for a neutral compound, the ion charge for a polyatomic ion.
Question
Define oxidation in terms of oxidation state.
Answer
An **increase** in oxidation state — the atom has **lost** electrons (OIL).
Question
Define reduction in terms of oxidation state.
Answer
A **decrease** in oxidation state — the atom has **gained** electrons (RIG).
Question
What does OIL RIG stand for?
Answer
**O**xidation **I**s **L**oss, **R**eduction **I**s **G**ain (of electrons).
Question
What is an oxidising agent?
Answer
The species that **takes** electrons and is itself **reduced** (its oxidation state goes down), e.g. O_{2}, Cl_{2}.
Question
What is a reducing agent?
Answer
The species that **gives** electrons and is itself **oxidised** (its oxidation state goes up), e.g. a reactive metal.
Question
How do you spot a redox reaction?
Answer
An atom's oxidation state **changes** during the reaction — so electrons have been transferred.
Question
Oxidation state of S in SO_{4}²⁻?
Answer
**+6** — four O at −2 (= −8) plus S must equal the ion charge −2, so S = +6.
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Topic 6.2 hub
Electron transfer reactions
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