Oxidation Number Rules

Group 1A metals

Always +1

Group 2A metals

Always +2

Group 7A / halogens

Usually −1; can be positive when bonded to oxygen or a more electronegative halogen

Fluorine

Always −1 in compounds

Oxygen

Usually −2; −1 in peroxides

Hydrogen

+1 with nonmetals and −1 with metals

Metals in ionic compounds

Usually have positive oxidation numbers

Nonmetals

Usually have negative oxidation numbers, but can have positive oxidation numbers when bonded to more electronegative elements

Free/uncombined elements

Oxidation number is always 0

Monatomic ion

Oxidation number equals the charge of the ion

Neutral compound

Sum of all oxidation numbers must equal 0

Polyatomic ion

Sum of all oxidation numbers must equal the charge of the ion

Transition metals

Can have multiple oxidation numbers; use the compound/ion to determine which one

Group 3A metals

Usually +3

Group 6A elements

Usually −2 when acting as anions; can have positive oxidation numbers when bonded to more electronegative elements