Chemistry - A Molecular Science

Chapter 6 Molecular Structure & Bonding

Figure 6.15 Hybrid orbitals and unused p orbitals The orientation of hybrid (blue) and unused p (grey) orbitals Table 6.2

Hybridization and arrangement of electron

groups around central atoms obeying the octet rule

number hybridi-

bond

π^

of groups zation

angle

bonds

2 sp 180

o^2

3 sp

2

120

o^1

4 sp

3

109

o^0

HH

H CCCH

AB

X

Z

Y

Figure 6.16 Coordinate system used in allene, C

H 3

(^4)
2sp+2p
3sp + 1p
2
4sp (no p)
3
(a)
(b)
(c)
Combining more than two orbitals involves more than two combinations, which are
done in a similar manner, but their construc
tion is beyond the scope of our discussion.
Thus, only the results are summarized here.

1. Two sp hybrid orbitals
   are produced by combining one s and one p orbital as shown in
   Figure 6.14. They are oriented 180
   o from one another as shown in Figure 6.15a. Only one p
   orbital is used in their construction, so two p orbitals are available to form
   π bonds. Thus, sp
   hybridized atoms form two
   π bonds (one triple bond or two double bonds) and have bond
   angles of 180
   o. Note that sp hybrid orbitals lie along the axis of the p orbital used to
   construct them, so if the
   σ bonds are directed along the z-axis, then the s and the p
   (^) z
   orbitals would be used to make the hybrid orbitals, and the p
   and px
   orbitals would be used y
   to form
   π bonds.


2. 
   

   Three sp
   2 (spoken “sp two”) hybrid orbitals
   are produced by combining one s and two p
   orbitals (Figure 6.15b). The three hybrid orbi
   tals lie in the plane defined by the two p
   orbitals used to construct them and are oriented 120
   o from one another. Two p orbitals are
   used in the hybridization, so only one p orbital remains to form a
   π bond. Thus, sp
   2
   hybridized atoms have one double bond and 120
   o bond angles

   
   

. If the hybrid orbitals lie in

the xy plane, then the p

and px

orbitals must be used to construct them, which leaves the y

p orbital available for z

π bonding.

3.

Four sp

3 (spoken “sp three”) hybrid orbitals

are produced by combining the s and all three p

orbitals (Figure 6.15c). There are no p orbitals available to form

π bonds, so sp

3 hybridized

atoms form only single bonds and have bond angles of 109

o.

The hybridization of an atom can be dete

rmined from the number of electron groups

around it because the number of hybrid orbitals must equal the number of electron groups. For example, an atom with four electron groups requires four hybrid orbitals, so the atom must be sp

3 hybridized. In addition, atoms that are sp or sp

2 hybridized have unused p

orbitals that are used to form

bonds, so the hybridization of an atom describes both the π

structure and bonding around the atom (Table 6.2). An atom that is sp

2 hybridized has

three electron groups, 120

o bond angles, and is involved in one

bond, while an sp π

hybridized atom has two electron groups, 180

o bond angles, and two

bonds. π

We conclude our discussion of valence bond theory by using it to explain why the

planes of the two CH

groups of allene (Figure 6.16) 2

are perpendicular. We start by

determining the hybridization of each carbon atom. The central atom is surrounded by two electron groups and is involved in two

bonds, so it is sp hybridized. If the bonding axis π

is the z-axis, then the p

orbital must be used to construcz

t the hybrid orbitals, which leaves

the p

and px

orbitals available to form the two y

bonds. Each of the terminal carbon atoms π

(C

and CA

) is surrounded by three electron groups and is involved in one B

bond, so each π

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