Oxidation numbers (w) are effective charge numbers related to the atom's electronegativity



(1) The sum of the oxidation numbers of all the atoms in the species is equal to the total charge

(2) For atoms in their elemental forms, w = 0

(3) For alkali metals (Li, Na, etc.), w = +1

(4) For alkaline earths (Mg, Sr, etc.), w = +2

(5) For Hydrogen: with non-metals w = +1, with metals w = -1

(6) For Fluorine, w = -1

(7) For Oxygen: w = -2 unless combined with F, w = -1 for peroxides (O₂^2-), and w = -1/2 for superoxides (O₂^-)

(8) All other halogens (Cl, Br, etc.), w = -1 unless combined with O

(9) Ionic binary species, w = charges per ion (ZnS, w(Zn) = +2, w(S) = -2)

Examples:

Species	w
O2	w(O) = 2(0)
Fe3+	w(Fe) = +3
Br-	w(Br) = -1
SO2	w(O) = 2(-2) = -4, w(S) = +4
SO42-	w(O) = 4(-2) = -8, w(S) = +6
H2S	w(H) = 2(+1) = 2, w(S) = -2
NO3-	w(O) = 3(-2) = -6, w(N) = +5
ClO3-	w(O) = 3(-2) = -6, w(Cl) = +5
Cu	w(Cu) = 0