Assigning Oxidation States
Oxidation states are a formal numbering system used to track how electrons are distributed among atoms in compounds and ions. They provide a clear way to identify which species are oxidised or reduced in a chemical reaction without needing to draw complex diagrams.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The oxidation state of an atom in an uncombined element is always 0 (e.g., O in O₂, Na in Na metal).
- For a simple ion, the oxidation state is equal to its charge (e.g., Mg in Mg²⁺ is +2, Cl in Cl⁻ is -1).
- The sum of all oxidation states in a neutral compound is 0. For a polyatomic ion, the sum equals the ion's overall charge.
- A fixed hierarchy of rules applies: Group 1 metals are +1, Group 2 are +2. Oxygen is usually -2 (except in peroxides like H₂O₂ where it's -1). Hydrogen is usually +1 (except in metal hydrides like NaH where it's -1).
- Oxidation is an INCREASE in oxidation state (becomes more positive).
- Reduction is a DECREASE in oxidation state (becomes more negative).
Formulae
Sum of oxidation states in a neutral compound = 0 When finding an unknown oxidation state in any neutral molecule, such as H₂SO₄ or CO₂.
Sum of oxidation states in a polyatomic ion = ion charge When finding an unknown oxidation state within a charged polyatomic ion, such as SO₄²⁻ or NH₄⁺.
Definitions
- Oxidation State
- A hypothetical charge assigned to an atom in a substance, assuming all bonds were completely ionic. It reflects the atom's degree of oxidation.
- Oxidation
- A chemical process where an atom's oxidation state increases, corresponding to a formal loss of electrons.
- Reduction
- A chemical process where an atom's oxidation state decreases, corresponding to a formal gain of electrons.
Worked example
Determine the oxidation state of manganese (Mn) in the permanganate ion, MnO₄⁻.
- 1
Identify the overall charge of the ion.
For MnO₄⁻, the charge is -1.
The sum of oxidation states must equal -1.
- 2
Assign the oxidation state for oxygen.
Following the rules, oxygen is -2.
- 3
Calculate the total charge contribution from all oxygen atoms.
There are four oxygen atoms, so the total is 4 × (-2) = -8.
- 4
Set up an algebraic equation.
Let the oxidation state of Mn be 'x'.
The equation is:
x + (-8) = -1 - 5
Solve for x.
x = -1 + 8, which gives x = +7 - 6
The oxidation state of manganese in MnO₄⁻ is +7.
Answer: +7
Common mistakes
- ×Making sign errors in the final calculation, for instance, forgetting that the overall charge of an anion is negative.
- ×Forgetting to multiply an atom's oxidation state by its subscript in the chemical formula (e.g., treating the four oxygens in MnO₄⁻ as a single -2 charge instead of a total of -8).
- ×Misremembering the oxidation states in common exceptions, such as oxygen in peroxides (O₂²⁻) being -1, or hydrogen in metal hydrides (H⁻) being -1.
No-calculator tips
- ✓Always write out the simple algebraic equation (e.g., 'x + 4*(-2) = -1') instead of doing it all in your head. This makes it easier to track negative numbers and avoid simple arithmetic mistakes.
- ✓Start by assigning numbers to the elements with the most definite rules first (e.g., Group 1, Group 2, H, O) and solve for the unknown element last.
- ✓After finding your answer, quickly plug it back into the formula to double-check that the sum of the oxidation states correctly equals the overall charge. This sanity check catches most calculation errors.