The Shapes of Molecules One of the important ways that molecular compounds differ from ionic compounds is in their structures. Ionic bonding is nondirectional in the sense that, at a given distance, an ion will attract others of the opposite sign with the same force regardless of where these other ions are located around the ion in question - there is no preferred direction. Because of this, the way ions arrange themselves in an ionic solid is determined just by the tendency to maximize attractions between ions of opposite charge and to minimize repulsions between like-charged ions. In molecular compounds, quite a different situation exists.
Covalent bonds are highly directional .
That is, for a given central atom in a molecule there are preferred orientations for the atoms attached to it, because covalent bonds are not formed with equal ease in all directions. As a result, in polyatomic molecules the atoms remain in the same relative orientation, regardless of whether the substance is a solid, liquid or a gas and we can say that the molecules have a definite structure or shape. The shapes of molecules are very important because many of their physical and chemical properties depend upon the three-dimensional arrangements of their atoms. For example, the functioning of enzymes, which are substances that control how fast biochemical reactions occur, requires that there be a very precise fit between one molecule and another. Even slight alterations in molecular geometry can destroy this fit and deactivate the enzyme, which in turns prevents the biochemical reaction involved from occurring.