Why do we not assign oxidation numbers to ionic compounds?

Alkali metal salts typically form a #I^+# ion; and alkaline earth salts typically form a #II^+# ion. Halogens, #"Group VII elements"#, typically form an #X^-# ion; and chalcogens, #"Group VI elements"#, typically form an #E^(2-)# ion.

Given these charges, an halide anion, will form a #MX# salt with the alkali metals, and an #M'X_2# salt with the alkaline earths. The salt must NECESSARILY be neutral. And likewise for the chalcogen salts, #M_2E#, and #M'E# etc.

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. #"Ferric salts"# of #Fe^(3+)#, and #"ferrous salts"# of #Fe^(2+)# are good examples. In this scenario the oxidation states must simply be known. More complex anions, e.g. #"chlorates"#, #"perchlorates"#, are sometimes distinguished by the oxidation state of the halogen: #"chlorate "=Cl(+V); "perchlorate "=Cl(+VII)#. Depending on where your are A level or undergrad, you have to make the determination as to what you need to know.

And thus the molecularity, the composition of a given ionic compound should be fairly easy to determine.