5 Steps to a 5 AP Chemistry

One place this type of bonding occurs is in the formation of the carbon-to-carbon double

bond, as will be discussed later.

sp^3 hybridizationresults from the mixing of one s orbital and three p orbitals, giving four

sp^3 hybrid orbitals with a tetrahedral geometric orientation. This sp^3 hybridization is found

in carbon when it forms four single bonds.

sp^3 d hybridizationresults from the blending of an s orbital, three p orbitals, and one d

orbital. The result is five sp^3 d orbitals with a trigonal bipyramidal orientation. This type of

bonding occurs in compounds like PCl 5. Note that this hybridization is an exception to the

octet rule.

sp^3 d^2 hybridizationoccurs when one s, three p, and two d orbitals are mixed, giving an octa-

hedral arrangement. SF 6 is an example. Again, this hybridization is an exception to the octet

rule. If one starts with this structure and one of the bonding pairs becomes a lone pair, then

a square pyramidal shape results, while two lone pairs gives a square planar shape.

Figure 11.8 shows the hybridization that occurs in ethylene, H 2 C=CH 2. Each carbon

has undergone sp^2 hybridization. On each carbon, two of the hybrid orbitals have over-

lapped with an s orbital on a hydrogen atom, to form a carbon-to-hydrogen covalent bond.

The third sp^2 hybrid orbital has overlapped with the sp^2 hybrid on the other carbon to form

a carbon-to-carbon covalent bond. Note that the remaining p orbital on each carbon that

has not undergone hybridization is also overlapping above and below a line joining the car-

bons. In ethylene there are two types of bond. In sigma(σ) bonds,the overlap of the

orbitals occurs on a line between the two atoms involved in the covalent bond. In ethylene,

the C–H bonds and one of the C–C bonds are sigma bonds. In pi(π) bonds,the overlap

of orbitals occurs above and below a line through the two nuclei of the atoms involved in

the bond. A double bond always is composed of one sigma and one pi bond. A carbon-to-

carbon triple bond results from the overlap of an sp hybrid orbital and two p orbitals on

one carbon, with the same on the other carbon. In this situation there will be one sigma

bond (overlap of the sp hybrid orbitals) and two pi bonds (overlap of two sets of p orbitals).

Molecular Orbital Theory

Still another model to represent the bonding that takes place in covalent compounds is the

molecular orbital theory. In the molecular orbital (MO)theory of covalent bonding,

atomic orbitals (AOs) on the individual atoms combine to form orbitals that encompass the

Bonding  155

H

H

H

H

CC

H1s H1s

sp^2

sp^2

sp^2 sp^2

sp^2

sp^2

H

C

Combined

unhybridized 2p

H H

H

C

Figure 11.8 Hybridization in ethylene, H 2 C=CH 2.