oxidation numbers

We are now going to look at some reactions that involve a transfer of electrons.

The oxidation number is the charge an atom in a substance would have if the pairs of electrons in each bond belonged to the more electronegative atom. Now this means that in a compound made up of monatomic ions, like NaCl, in which the bonding pairs do belong to the more electronegative atom, the oxidation number equals the ionic charge. The sodium ion has an oxidation number of +1 while the chlorine ion has an oxidation number of -1. Here are some rules that are helpful in finding the oxidation number of a specific atom:
  • The oxidation number of an atom in an elementary substance is 0. This means that the oxidation number of an O atom in O2 is 0.
  • The oxidation number of a Group IA atom in any compund is +1; The oxidation number of a Group IIA atom in any compund is +2.
  • The oxidation number of fluorine is -1 in all of its compounds.
  • The oxidation number of chlorine, bromine, and iodine is -1 in any compound containing only two elements.
  • The usual oxidation number of oxygen in a compound is -2. The major exceptions of this rule are peroxides, like H2O2, which have an oxidation number of -1.
  • The oxidation number of hydrogen in most compounds is +1.
  • The sum of the oxidation numbers in a compound is always zero. For something that is an ion consisting of two atoms (a polyatomic ion), the oxidation numbers add up to the charge on the ion.

rules that are helpful in finding the oxidation number of a specific atom


You might be heartily confused at this point as to what oxidation numbers are and how to find them. Don't worry, you can usually find the oxidation numbers of elements in ionic compounds by looking at the individual ions in the compound.

example:
In H2SO4, H = +1 and O = -2.

Since there are 4 oxygen atoms, the total charge from oxygen is -8. Therefore the charge of S must be +6, since the net charge of the molecule is zero.