CK-12-Chemistry Intermediate

9.2. Molecular Geometry http://www.ck12.org

FIGURE 9.22

Bromine pentafluoride, BrF 5.

The six groups of electrons give it an octahedral domain geometry, but two of the atoms are replaced by two lone
pairs. In order to maximize the space available to each of the lone pairs, they occupy positions that are directly
opposite each other. As a result, the four remaining fluorine atoms are in the same plane as the xenon atom, and all
F-Xe-F angles are equal to 90°. This molecular geometry is calledsquare planar.

FIGURE 9.23

Xenon tetrafluoride, XeF 4.

Summary of VSEPR

The VSEPR model can be applied to predict the molecular geometry of a given molecular compound. The steps to
follow can be summarized as follows:

• Draw the Lewis electron dot structure for the molecule.


• Count the total number of electron pairs around the central atom. This is referred to as theelectron domain
  geometry.


• If there are no lone pairs around the central atom, refer to the table below (Table9.2), to determine the
  molecular geometry, which is the same as the electron domain geometry.


• If there are one or more lone pairs on the central atom, the molecular geometry (the actual shape of the
  molecule) will not be the same as the electron domain geometry. Refer to the table below (Table9.3).


• In predicting bond angles, remember that a lone pair takes up more space than a bonding pair or pairs of
  electrons.

TABLE9.2: Geometries of Molecules and Ions in Which the Central Atom Has No Lone Pairs

Number of Electron Pairs

Around Central Atom

Electron Domain Geome-

try

Molecular Geometry Examples

2 linear linear BeCl 2 , CO 2

3 trigonal planar trigonal planar BF 3 , CO 32 −