Fractional oxidation numbers are sometimes used to describe the average oxidation state of several atoms in a compound. It is important to understand that not all atoms of a particular element must have the same oxidation number in a compound.
Let's take, for example, propane. Propane's molecular formula is
This represents a fractional oxidation number. However, if you take into account the fact that propane's structure is actually
you will see that, in this case, the first
It is very important to try and avoid fractional oxidation numbers, since oxidation numbers are theoretical values used to do a sort of electron bookkeeping; oxidation numbers allow you to compare how many electrons an atom "owns" in a molecule or an ion, as opposed to how many valence electrons it has by itself.
Another example of fractional oxidation states in a compound is
This indeed represents an average oxidation state for the
So, if you have an ionic compound, you can keep the different oxidation states of an atom separated by fragmenting the ionic compound into its respective ions.
Organic compounds like propane can be a little tricky at first, but writing their structural molecular formulas will allow you to determine the oxidation number of, for example, each