Oxidation numbers and electron transfer
≈ 48 minOxidation numbers and electron transfer
Oxidation numbers are accounting values that help track electron transfer in redox reactions. Elements in their uncombined form have oxidation number zero. In a neutral compound, the sum of oxidation numbers is zero; in an ion, the sum equals the ion charge. An increase in oxidation number means oxidation, and a decrease means reduction. These definitions work even when oxygen is not visibly added or removed. In a full redox equation, identify both changes and verify that the electrons lost equal the electrons gained before claiming it is balanced.
Work it through
In SO₄²⁻, oxygen is usually −2. Four oxygens total −8. The ion has charge −2, so sulfur must be +6 to make the sum −2. If sulfur later changes from +6 to +4, it has been reduced because its oxidation number decreased. That conclusion is stronger than relying on a mnemonic alone.
Mastery target
Assign simple oxidation numbers, identify both oxidation and reduction, and use electron accounting to justify a redox conclusion.
What happens to a species that gains electrons?
Name the key chemistry term from Oxidation numbers and electron transfer that best fits the explanation and visual model.
What is the oxidation number of sulfur in SO₄²⁻, using oxygen = −2?
Which statement corrects a common misunderstanding in Oxidation numbers and electron transfer?