Why do we not assign oxidation numbers to ionic compounds?
Because MOST of the time, the identity of the ionic compound is specified by the regular oxidation states expressed by each ion in the compound.
Alkali metal salts typically form a
Given these charges, an halide anion, will form a
All we have done here is to balance charge appropriately.
On the other hand, there are some situations where the oxidation state of the metal is ambiguous, in that the metal can adopt 2 oxidation states.
And thus the molecularity, the composition of a given ionic compound should be fairly easy to determine.