Oxidation Number Rules

H rule

Hydrogen is +1 (except in metal hydrides where it is -1)

O rule

Oxygen is usually -2 (except in peroxides where it is -1, and in OF₂ where it is +2)

Group 1A metals

Always +1 in compounds

Group 2A metals

Always +2 in compounds

Fluorine

Always -1 in compounds

Halides (Cl, Br, I)

Usually -1 unless combined with oxygen or other halogens

Sum of oxidation numbers

Must equal overall charge of molecule or ion

Free elements

Oxidation number is 0

Monoatomic ions

Oxidation number equals the ion charge

Special compounds

Some compounds like peroxides, superoxides, and OF₂ have exceptions to the usual oxygen rules

Identifying REDOX

Oxidation numbers increase in oxidation, decrease in reduction

Tip

Assign oxidation numbers first, then identify what is oxidized and reduced