Oxidation number (or state)

1.    The oxidation number of an element is zero(0).

Cu(s) Mg(s) O2(g) S8(s) Hg(l)
ON  0  0  0  0  0
2.    For monatomic ions, the oxidation number is the same as the charge on the ion.
Na+(aq) Cu2+(aq) Fe3+(aq) Cl-(aq) S2-(aq) N3-(aq)
ON +1 +2 +3 -1 -2 -3
3.    In binary compounds (or polyatomic ions), the second element is assigned a negative
       oxidation number, usually the same as the charge on its monatomic ion.
SCl2 SiBr4 CS2 P4S4 CSe2 CN- S2N+
ON    -1     -1   -2    -2   -2   -3    -3

a.    F always -1.

OF2 CF4 BF4- ClF6+
ON   -1   -1   -1    -1
b.    O usually -2 (except peroxide -1 and superoxide -1/2).  Peroxides and superoxides
       can only be determined if the cation they are associated with has a fixed charge (H,
       alkali metals or alkaline-earth metals).
H2O P2O5 VO2+ UO22+ ClO3- SO42- PO43-
ON     -2     -2   -2   -2    -2   -2   -2

 
H2O2 BaO2 Na2O2 KO2
ON +1 -1 +2 -1 +1 -1 +1 -1/2
c.    Hydrogen is -1 only when combined with a metal, hydride ion.  H is +1 when
       written first (usually acids).
LiH CaH2 AlH4-
ON    -1     -1    -1

 
H2O HCl H2SO4 H3PO4
ON +1 +1 +1 +1
4.    The sum of oxidation numbers in a compound or polyatomic ion is equal to the overall
       charge on the compound (0) or the ion.
SCl2 SiBr4 CS2 P4S4 CSe2 CN- S2N+
ON +2 -1 +4 -1 +4 -2 +2 -2  +4 -2 +2 -3 +2 -3 

 
OF2 CF4 BF4- ClF6+
ON +2 -1 +4 -1 +3 -1 +7 -1

 
H2O P2O5 VO2+ UO22+ ClO3- SO42- PO43-
ON +1 -2 +5 -2 +4 -2 +6 -2 +5 -2 +6 -2 +5 -2

 
LiH CaH2 AlH4-
ON +1 -1 +2 -1 +3 -1

 
H2O HCl H2SO4 H3PO4
ON +1 -2 +1 -1 +1 +6 -2 +1 +5 -2


These rules do work as simply with molecular compounds with more than two types of atoms.

For example, C2H6O2, and many other organic compounds.