9.2. Molecular Geometry http://www.ck12.org
FIGURE 9.15
bipyramidal, or octahedral. However, when one or more of the bonding pairs of electrons is replaced with a lone
pair, the molecular geometry, or actual shape of the molecule, is altered. In keeping with the A and B symbols
established in the previous section, we will use E to represent a lone pair on the central atom (A). A subscript will
be used when there is more than one lone pair. Lone pairs on the surrounding atoms (B) do not affect the geometry.
AB
The Lewis structure for ozone consists of a central oxygen atom that has a double bond to one of the outer oxygen
atoms and a single bond to the other.
This leaves one lone pair on the central atom, and the molecule displays resonance. Since VSEPR does not
distinguish between double and single bonds, the resonance does not affect the geometry. Molecules with three
electron pairs have a domain geometry that is trigonal planar. Here, the lone pair on the central atom repels the
electrons in the two bonds, causing the atom to adopt abentmolecular geometry.
FIGURE 9.16
Ozone, O 3.One might expect the O-O-O bond angle to be 120°, as in a trigonal planar molecule. However, within the context
of the VSEPR model, lone pairs of electrons can be considered to be slightly more repulsive than bonding pairs of
electrons, due to their closer proximity to the central atom. In other words, lone pairs take up more space. Therefore
the O-O-O angle is slightly less than 120°.
AB
The ammonia molecule contains three single bonds and one lone pair on the central nitrogen atom.