Free elements have an oxidation state of 0

i.e. Na = 0 and Cl₂=0 in 2 Na(s) + Cl₂(g)

Monatomic ions have an oxidation state equal to their charge.

i.e. Na = +1 and Cl = -1 in NaCl;

(+1) + (-1) = 0

The sum of oxidation states in a compound is 0.

i.e. Na = +1 and Cl = -1 in NaCl;

(+1) + (-1) = 0

The sum of oxidation states of all the atoms in a polyatomic ion equals the charge of the ion.

i.e. N = +5 and O = -2 in NO₃^-;

(+5) + 3(-2) = -1

Group 1 metals have an oxidation state of +1 in all their compounds.

i.e. Na = +1 in NaCl

Group 2 metals have an oxidation state of +2 in all their compounds

i.e. Mg = +2 in MgCl₂

In their compounds, nonmetals have oxidation states according to the table below. (nonmetals higher on the table take priority)

F = -1

H = +1

O = -2

Group 7a = -1

Group 6a = -2

Group 5a = -3