5 PCl 5 trigonal
bipyramidal90°,120°,180°6 SF 6 octahedral 90°,180°While the number of electron pairs dictates their overall arrangement around the central atom, it is
only a starting point in arriving at the actual description of the geometry of the molecule. If one of
the X’s in the table above is a lone pair of electrons rather than an actual atom or group of atoms,
new terms need to be introduced to describe the spatial arrangement of the atoms. The example
below illustrates this point.
Example: Predict the geometry of NH 3.
Solution:In describing the shape of a molecule, only the arrangement of atoms (not electrons) is considered.
Even though the electron pairs are arranged tetrahedrally, the shape of NH 3 is described as trigonal
pyramidal. It is not trigonal planar because the lone pair repels the three bonding electron pairs,
causing them to move as far away as possible.
Example: Predict the geometry of CO 2.1. The Lewis structure of NH 3 is:2. The central atom, N, has 3 bonding electron pairs and 1 nonbonding electron pair, for a total of 4 electron pairs.
3. The 4 electron pairs will be farthest apart when they occupy the corners of a tetrahedron.