5 Steps to a 5 AP Chemistry

This determination of the molecular geometry of carbon dioxide and water also

accounts for the fact that carbon dioxide does not possess a dipole and water has one, even

though both are composed of polar covalent bonds. Carbon dioxide, because of its linear

shape, has partial negative charges at both ends and a partial charge in the middle. To pos-

sess a dipole, one end of the molecule must have a positive charge and the other a negative

end. Water, because of its bent shape, satisfies this requirement. Carbon dioxide does not.

Valence Bond Theory

The VSEPR theory is only one way in which the molecular geometry of molecules may be

determined. Another way involves the valence bond theory. The valence bond theory

describes covalent bonding as the mixing of atomic orbitals to form a new kind of orbital,

a hybrid orbital. Hybrid orbitalsare atomic orbitals formed as a result of mixing the

atomic orbitals of the atoms involved in the covalent bond. The number of hybrid orbitals

formed is the same as the number of atomic orbitals mixed, and the type of hybrid orbital

formed depends on the types of atomic orbital mixed. Figure 11.7 shows the hybrid orbitals

resulting from the mixing of s, p, and d orbitals.

sp hybridizationresults from the overlap of an s orbital with one p orbital. Two sp hybrid

orbitals are formed with a bond angle of 180º. This is a linear orientation.

sp^2 hybridizationresults from the overlap of an s orbital with two p orbitals. Three sp^2

hybrid orbitals are formed with a trigonal planar orientation and a bond angle of 120º.

154  Step 4. Review the Knowledge You Need to Score High

CO O

Carbon Dioxide

H

H

O

Water

Figure 11.6 Lewis structures of carbon dioxide and water.

Linear

Trigonal

planar

Trigonal

Tetrahedral bipyramidal Octahedral

Atomic orbitals

mixed

Hybrid orbitals

formed

Unhybridized

orbitals remaining

Orientation

one s

one p

two sp

two p

one s

two p

three sp^2

one p

one s

three p

one d

five sp^3 d

four d

one s

three p

two d

six sp^3 d^2

three d

one s

three p

four sp^3

none

Figure 11.7 Hybridization of s, p, and d orbitals.

KEY IDEA