6. Why are ionic substances not known as molecules?

Answer: ionic compounds are not known as molecules because they contain aggregate of positive and negative charges which arise from the transfer of electron from the metallic atom to the non-metallic atom. But in covalent substance, there is a sharing of electron pair between the atoms involved and this makes them to form a molecules

So, an ionic compound actually has two (or even more) different species in a single crystal or solution, which exist independently. The only reason why they are together is because of the total net charge, which must be zero.