http://www.ck12.org Chapter 9. Covalent Bonding
FIGURE 9.10
Ammonia, NH 3.Recall that the bond angles in the tetrahedral CH 4 molecule are all equal to 109.5°. One might expect the H-N-H
bond angles in ammonia to be 109.5° as well, but slight adjustments need to be made for the presence of lone pairs.
Within the context of the VSEPR model, lone pairs of electrons are 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 H-N-H angle is slightly less than 109.5°. Its actual value is approximately 107°.
Water
A water molecule consists of two bonding pairs and two lone pairs.
The water molecule, like the ammonia and methane molecules, has a tetrahedral domain geometry. In the water
molecule, two of the electron pairs are lone pairs rather than bonding pairs. The molecular geometry of the water
molecule is referred to asbent. The H-O-H bond angle is 104.5°, which is smaller than the bond angle in NH 3.
FIGURE 9.11
Water, H 2 O.Summary of VSEPR
The VSEPR model can be applied to predict the molecular geometry of a given molecular compound. There are a
number of additional shapes that can be constructed starting from other electron domain geometries and replacing
one or more atoms with lone pairs. We will not go over each individual case in this book, but the names for various
shapes are provided in the tables below as a reference. To determine the shape of a given molecule, use the following
steps: