Untitled - Kelly Walsh High School

146 CHEMISTRY FOR THE UTTERLY CONFUSED
Get Started
The main goal of this chapter is to help you master electron and molecular geometry
and hybridization. This will lead to information on the polarity of molecules.
You might want to review Section 7-5 on electron configuration. Section 9-7 on
writing Lewis structures is also important. Don’t Forget!—Practice, Practice,
Practice.
10-1 Molecular Geometry (VSEPR)
In Chapter 9, we indicated that the Lewis structure showed the bonding pattern
of a molecule, but not necessarily its shape. The shape of a molecule greatly
influences its properties. One method to predict the shape of molecules is the
valence-shell electron pair repulsion (VSEPR) theory. The basis of this theory
is that the valence shell electron pairs around a central atom will try to move as
far away from each other as possible. This includes electrons in bonds and
elsewhere. The electrons do this to minimize the repulsion between the like
(negative) charges. We will actually determine two geometries. The first is the
electron-group (pair) geometry. The electron group geometry considers all electron
pairs surrounding a nucleus. The second is the molecular geometry. In this
case, the nonbonding electrons (electron lone pairs) become “invisible.” We
consider only the arrangement of the atomic nuclei. For the purposes of geometry,
double and triple bonds count the same as single bonds.
In order to determine the electron-group and molecular geometry:
1. Write the Lewis electron-dot formula of the compound.
2. Determine the number of electron pair groups surrounding the central atom(s).
Remember that double and triple bonds count the same as a single bond.
3. Determine the geometric shape that maximizes the distance between the
electron groups. This is the electron-group geometry.
4. Mentally allow the nonbonding electrons to become invisible. They are still
present and are still repelling the other electron pairs. However, we just
don’t “see” them. We then determine the molecular geometry from the
arrangement of bonding pairs around the central atom.
Figure 10-1 shows the electron-group and molecular geometry for 2 to 6 electron
pairs.