Untitled - Kelly Walsh High School

148 CHEMISTRY FOR THE UTTERLY CONFUSED
For example, let’s determine the electron-group and molecular geometry of carbon
dioxide, CO 2, and water, H 2O. At first glance, you might imagine that the
geometry of these two compounds would be similar since both have a central
atom with two groups (atoms) attached. However, let’s see if that is true.
First, write the Lewis structure of each. Figure 10-2 shows the Lewis structures
of these compounds.
δ− δ+ δ− δ−
O C O
O
H
δ+
H
δ+
Figure 10-2 Lewis structure of carbon
dioxide and water
Next, determine the electron group geometry of each. For carbon dioxide, there
are two electron groups around the carbon. Two electron pairs is a linear structure.
For water, there are four electron pairs around the oxygen, two bonding,
and two nonbonding electron pairs. The presence of four total pairs gives tetrahedral
electron-group geometry.
Finally, mentally allow the nonbonding electron pairs to become invisible.
What remains is the molecular geometry. For carbon dioxide, all groups are
involved in bonding. There are no “invisible” groups. This means the electrongroup
and the molecular geometry are the same (linear). However, water has
two nonbonding pairs of electrons (now invisible). The remaining bonding electron
pairs (and hydrogen nuclei) are in a bent molecular arrangement. The
angle between these pairs is similar to that in the tetrahedral structure. The
presence of lone pairs makes the angle slightly smaller.
Determining the molecular geometry of carbon dioxide and water also explains
why their polarities are different. Carbon dioxide is not polar and water is. This
is true even though both are composed of polar covalent bonds. To be a polar
molecule, one end must have a partial positive charge (d) and the other must
have a partial negative charge (d). Carbon dioxide, because of its linear shape,
has partial negative charges at both ends and a partial positive charge in the
middle. Water, because of its bent shape, has a partial negative end, the oxygen,
and a partial positive end, the hydrogen side. Carbon dioxide does not have a
partial positive end. The polarity of the molecule is important because polar
molecules attract other polar molecules. This may dramatically affect the properties
of that substance. This is true with water. (See Chapter 11 for a discussion
of water and intermolecular forces.)