oxidation state
Summary
Oxidation state (oxidation number) is the hypothetical charge an atom would have if every bond's shared electrons went to the more electronegative atom. Oxidation is a rise in oxidation state and reduction is a fall, so it shows what is oxidized or reduced in a reaction.
Detail
Meaning: Oxidation state is a bookkeeping charge, found by giving each bond's electrons to the more electronegative atom. Common rules: an atom in a pure element is 0; a monatomic ion equals its charge; fluorine is −1; group 1 metals are +1 and group 2 metals are +2; hydrogen is usually +1 (−1 in metal hydrides); oxygen is usually −2 (−1 in peroxides). A rise means loss of electrons (oxidation); a fall means gain (reduction).
Equation: Σ(oxidation states of all atoms) = overall charge of the species (oxidation states are formal, dimensionless numbers). Example: in MnO₄⁻, Mn + 4(−2) = −1, so Mn = +7.
On the MCAT: Redox passages give a reaction; assign oxidation states to find what is oxidized and reduced. The species that is reduced is the oxidizing agent, and the species that is oxidized is the reducing agent. In organic chemistry, alcohol → aldehyde → carboxylic acid raises the carbon's oxidation state.
Common confusion: Oxidation state is not always a real charge. In covalent molecules such as CO₂, carbon is assigned +4 but does not carry a full +4 charge. Also, the oxidizing agent is the species that gets reduced, not oxidized.
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.