The constituent units which make up an ionic solid are oppositely charged ions. Such solids or crystals are formed between constants which have very large differences in electron attracting power so as to allow complete transfer of electron form one constituent to another. These + ve and – ve ions are packed together in the crystal in such a way that unlike charges come as close as possible and like charges keep apart as far as permissible. Such an arrangement leads to a maximum attraction and minimum repulsion in the crystal. This arrangement of ions in the crystal is governed by columbic forces between ions and their environments. Consequently ionic compounds acquire physical properties which are significantly different from those of covalent compounds. We know that covalent molecules are mostly low melting and low boiling molecules like H₂, O₂, CO₂, halogens, some element halides, organic compounds, etc or large molecules and boiling points and are very hard and brittle. Then there are substances like H₂O, OF₂, SIF₄, PX₃ (X = halogens_, some transition and non-transition metal derivatives, etc., which pre polar and their properties lie between covalent and ionic compounds. Such substances are designated as predominantly covalent or pre- dominantly ionic. Their physical properties lie in between those of covalent and ionic compounds.

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