Molecular Geometry

Identifying types of bonds and lone pairs of electrons is also critical in accurately predicting the shape of a molecule. Electron pairs assume the position of bonds about a central atom and, because of their charge density, can actually take up more space than the electrons in a covalent bond. The molecular shape of ammonia (NH₃) provides an example of the influence of electron pairs. In each molecule there are three hydrogen atoms bonded to the central nitrogen atom. In addition, the central nitrogen atom also carries a lone electron pair. As with the case of methane, these four sets of electron pairs (three pairs participating in covalent nitrogen-hydrogen bonds and the one lone pair on the nitrogen atom) experience minimum electrical repulsion when arranged so that their bond angles approximately point toward at the four corners of a tetrahedron. The lone electron pair, however, has a higher charge density (charge per unit of space) and therefore exerts a greater electrical repulsion. The net effect of this increased repulsion by the lone pair means that, in the presence of a lone pair of electrons, the other bonds are forced to crowd together a bit to make additional space available to the lone electron pair. As a result, in ammonia the bond angles between the central nitrogen atom and the three hydrogen atoms are about 107° and the molecule becomes a pyramidal molecule (i.e., the bonds between the central nitrogen and the three surrounding hydrogen atoms are pointed at the corners of the base of a triangular pyramid). It is sometimes useful to envision the lone electron as pointing toward the apex of the pyramid.

Water (H₂O) has two oxygen-hydrogen covalent bonds. In addition, there are two electron pairs on the central oxygen molecule. In the same way that the lone electron pair on nitrogen distorts the bond angles in ammonia, the lone electron pairs on the oxygen atom in a water molecule force the two covalent bonds between oxygen and hydrogen to assume a bond angle of approximately 105° to form what is termed as a "bent molecule." Bent molecules such as water produce polar molecules if, as in the case of water, the bonded atoms have different electronegativity values.