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Chemistry · Grades 9–12

Oxidation states and redox bookkeeping

Balance electron accounting without confusing oxidation state with literal charge in every bond.

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Learn the idea

Rules

For a neutral compound, oxidation states sum to zero; for a polyatomic ion, they sum to its charge.

Oxidation

Oxidation is an increase in oxidation state; reduction is a decrease.

Common values

Oxygen is usually −2 and hydrogen usually +1 in simple school examples, but exceptions exist.

Bookkeeping

Oxidation state is a formal electron-accounting model, not always the real charge on an atom.

Worked examples

In neutral MgO, oxygen is assigned −2. What is magnesium’s oxidation state?

The sum must be zero, so Mg is +2.

2

In CO₂, each oxygen is −2. What is carbon’s oxidation state?

x + 2(−2) = 0 gives x = +4.

4

Check your understanding

  1. A metal changes from oxidation state 0 to +2. It is what?

    1. Unchanged.
    2. Oxidized.
    3. Reduced.

  2. An ion changes from +3 to +2. It is what?

    1. Oxidized.
    2. Reduced.
    3. Necessarily neutral.

  3. In SO₄²⁻ with oxygen −2, find sulfur’s oxidation state.

Answers and marking notes

Answers and marking notes
  1. Oxidized.

    An increase in oxidation state is oxidation.

  2. Reduced.

    The oxidation state decreases.

  3. 6

    x + 4(−2) = −2 gives x = +6.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.