16.3Oxidation States: Electron Bookkeeping

The idea

• Assign oxidation states with the standard rules.
• Use the change in oxidation state to spot redox.

An oxidation state is a bookkeeping charge, not always a real ionic charge. Rules, in priority order: an element in its elemental form is 0. A monatomic ion equals its charge. Fluorine in compounds is −1. Oxygen is −2 except in peroxides, where it is −1. Hydrogen is +1 when bonded to nonmetals and −1 in metal hydrides. The sum of oxidation states equals the charge of the species. Apply the rules from the top and let the unknown element be whatever makes the sum work.

If an element’s oxidation state goes up, it is oxidized. If it goes down, it is reduced. In CH₄ + 2 O₂ → CO₂ + 2 H₂O, carbon goes from −4 to +4 and is oxidized; oxygen goes from 0 to −2 and is reduced. A double-replacement precipitation usually shows no changes, which is how you prove it is not redox. Practice on polyatomic ions until sulfate, nitrate, and chlorate feel automatic.

Worked path

Assign oxidation states in H₂O₂, a peroxide, and say whether oxygen follows its usual rule.

1. Observe

   Hydrogen bonded to a nonmetal is +1. The molecule is neutral.

Check yourself