Oxidation numbers

The oxidation number is related to the charge on an atom. It is a useful index to help in recognizing what speciaies are gain or losing electrons in a chemical reaction.

Monatomic Species

For monatomic species, the rules are simple. The oxidation umber is the charge on the atom. Consider the following table.
Species Oxidation Number
Ar 0
Cl- -1
Na+ +1
Cr+3 +3
S-3 -3

Polyatomic Species

For polyatomic species that are pure elements, the oxidation number is always zero.
Species Oxidation Number
H2 0
O2 0
Cl2 0
P4 0

For species that are not pure elements, the oxidation umber is calculated relative to a few conventional definitions. For the following elements in polyatomic species, the following oxidation numbers are always assigned:
Species Oxidation Number
H +1
O -2
F -1

Other halogens are generally also assigned an oxidation number of -1 unless they are the central atoms in polyatomic oxyanion species.

To calculate the oxidation number of the central atom in a polyatomic ion, remember that the sum of the oxidation numbers must equal the charge on the ion. (For neutral species, the charge is zero.) Here are some examples.
What is the oxidation number on carbon in the CO3-2 ion?
Solution: 
      (#C) + 3(#O) = -2
      (#C) + 3(-2) = -2
      (#C)         = +4

What is the oxidation number on ccopper in the CuCl4-2 ion?
Solution: 
      (#Cu) + 4(#Cl) = -2
      (#Cu) +  4(-1) = -2
      (#Cu)         = +2

What is the oxidation number on nitrogen in NH3?
Solution: 
      (#N) + 3(#H) = 0
      (#N) + 3(+1) = 0
      (#N)         = -3

What is the oxidation number on boron in B2H6?
Solution: 
      2(#B) + 6(#H) = 0
      2(#B) + 6(+1) = 0
      2(#B)        = -6
      (#B)         = -3

Here are some for you to try:

What is the oxidation number on boron in the BO3-3 ion?
+3 +2 +1 -2

What is the oxidation number on phosphorus in the PO4-3 ion?
+7 +5 +3 -3

What is the oxidation number on sulfur in the SF6?
+8 +6 +4 -2

What is the oxidation number on Cl in the HOCl?
+5 +3 +1 -1

What is the oxidation number on sulfur in the sulfate ion (SO4-2)?
+6 +4 +2 -2

What is the oxidation number on sulfur in the thiosulfate ion (S2O3-2)?
+6 +4 +2 -2

Patrick E. Fleming
Department of Chemistry and Biochemistry
California State University, East Bay
patrick.fleming@csueastbay.edu