the threeclosest other pairs (the pairs bonding three F atoms in equatorial positions) and
180° from the other axial pair. If it were in an equatorial position, only the twoaxial pairs
would be at 90°, and the other two equatorial pairs would be less crowded at 120° apart.
The lone pair would be less crowded in an equatorialposition. The four F atoms then
occupy the remaining four positions. We describe the resulting arrangement of atomsas
a seesaw arrangement.Imagine rotating the arrangement so
that the line joining the two axial
positions is the board on which the
two seesaw riders sit, and the two
bonded equatorial positions are the
pivot of the seesaw.
334 CHAPTER 8: Molecular Structure and Covalent Bonding Theories
As we saw in Sections 8-8 and 8-9, the differing magnitudes of repulsions involving
lone pairs and bonding pairs often result in observed bond angles that are slightly different
from idealized values. For instance, lp/bprepulsion in the seesaw molecule SF 4 causes
distortion of the axial SXF bonds away from the lone pair, to an angle of 177°; the two
equatorial SXF bonds, ideally at 120°, move much closer together to an angle of 101.6°.
By the same reasoning, we understand why additional lone pairs also take equatorial
positions (AB 3 U 2 with both lone pairs equatorial or AB 2 U 3 with all three lone pairs equa-
torial). These arrangements are summarized in Figure 8-2.aee eaFFSF FS FeFae
FFeaForAB 4 U 4 bonded atoms (B)1 lone pair (U) in
equatorial position
Seesaw molecular geometry
Example: SF 4AB 3 U 2 3 bonded atoms (B)2 lone pairs (U) in
equatorial positions
T-shaped molecular geometry
Examples: ClF 3 , BrF 3AB 2 U 3 2 bonded atoms (B)3 lone pairs (U) in
equatorial positions
Linear molecular geometry
Examples: XeF 2 , I 3 –Figure 8-2 Arrangements of bonded atoms and lone pairs (five regions of high electron
density—trigonal pyramidal electronic geometry).