# 7.4: Molecules with Lone Pairs

The VSEPR theory is able to explain and predict the shapes of molecules which contain lone pairs. In such a case the lone pairs as well as the bonding pairs are considered to repel and avoid each other. For example, since there are two bonds in the SnCl2 molecule, one might expect it to be linear like BeCl2. If we draw the Lewis diagram, though, we find a lone pair as well as two bonding pairs in the valence shell of the Sn atom:

A lone pair also affects the structure of ammonia, NH3. Since this molecule obeys the octet rule, the N atom is surrounded by four electron pairs:

If these pairs were all equivalent, we would expect the angle between them to be the regular tetrahedral angle of 109.5°. Experimentally, the angle is found to he somewhat less, namely, 107°. Again this is because the lone pair is “fatter” than the bonding pairs and able to squeeze them closer together.

The electronic structure of the H2O molecule is similar to that of NH3 except that one bonding pair has been replaced by a lone pair:

Example $$\PageIndex{1}$$ : Molecular Geometry

Sketch and describe the geometry of the following molecules: (a) GaCl3, (b) AsCl3, and (c) AsOCl3.

Solution

a) Since the element gallium belongs to group III, it has three valence electrons. The Lewis diagram for GaCl3 is thus

Since there are three bonding pairs and no lone pairs around the Ga atom, we conclude that the three Cl atoms are arranged trigonally and that all four atoms are in the same plane.

b) Arsenic belongs to group V and therefore has five valence electrons. The Lewis structure for AsCl3 is thus

Since a lone pair is present, the shape of this molecule is a trigonal pyramid, with the As nucleus a little above an equilateral triangle of Cl nuclei.

c) The Lewis diagram for AsOCl3 is similar to that of AsCl3.

Since there are four bonding pairs, the molecule is tetrahedral. Sketches of each of these molecules are

The VSEPR theory can also be applied to molecules which contain five and six pairs of valence electrons, some of which are lone pairs. We have not included such species here because the majority of compounds fall into the categories we have described.