Oxidation numbers are similar to the valences or charges on
ions. The difference is that in covalent compounds (and polyatomic ions), the
electrons are not actually traded. Remember they are shared. So
the oxidation number is the charge that a chemical would have if it were
acting as an ion. The free ion may never exist in nature, but in a compound (or
poly atomic ion) it acts as if it had this charge.
How to assign oxidation numbers
- All elements that are not combined with anything have an oxidation state =
0. Ex. Au(s)= 0
- Oxidation number of a simple (one atom) ion is its charge. Group I
elements = +1, Group II elements = +2, Group 3 = +3 usually. Halogens are -1
Unless combined with a more electronegative
element. Ex. Al+3= +3 and
Cl- = -1.
- Fluorine in a compound is always -1. Ex. F in NaF =
- Oxygen has and oxidation number of -2 except in peroxides (H2O2).
Ex. O in Fe2O3 = -2.
- Hydrogen has an oxidation state of +1. Except in hydrides (which we won't
deal with in this class) Ex. H in H2SO4
- The sum of the oxidation numbers for any neutral compound = 0. Ex.
- In a compound, give the more electronegative element the charge it would
have if it were an ion. Ex. O in SO3
= -2, this means S = +6!!
- The sum of oxidation numbers for a polyatomic ion = the charge on the ion.