Chemistry - A Molecular Science

6.1 increases with the number of lone pairs.

We will indicate that the bond angle deviates

from the predicted value with a '~' in front of the angle.

~109

o

~120

o

180

o

a) AX

linear

2

b) AX E

bent

2

c) AX E

bent

22

CO

SO

H O

222

Figure 6.2 Shapes of molecules with three atoms Green lobes represent lone pairs.

o 120

~109

o

109

o

a) AX

b) AX E

c) AX

planar

pyramidal

tetrahedral

33

4

SO

NH

CF

334

Figure 6.3 Shapes of molecules with four and five atoms

The shapes shown in Figure 6.1 show the orientations that can be adopted by the

electron groups surrounding a central atom that obeys the octet rule. However, we typically determine the positions of only the atoms, not the lone pairs. Thus, a

molecular

shape

describes the shape adopted by only the at

oms. The lone pairs help establish what

that shape is, but the name of the shape appli

es only to that taken by the atoms. We now

consider the shapes of some generic molecu

les. The analysis we will follow has three

steps:

1. Determine the number of electr

on groups around the central atom.

2. Determine which of the electron distributions shown in Figure 6.1 applies.
   3. Name the molecular shape adopted by the atoms.
   We limit the discussion to molecules with two, three or four atoms (X) attached to one central atom (A), which may have one or more lone pairs (E). The possibilities for molecules in which A obeys the octet rule are discussed below and summarized in Table 6.1.

AX

: The A atom is surrounded by two electron gr 2

oups that orient as shown in Figure 6.1a.

Three atoms bound by these two electron

groups lie in a straight line, so AX

molecules are 2

linear

(Figure 6.2a) with an X-A-X bond angle of 180

o.

AX

E: The A atom is surrounded by three electron regions, two bonding and one lone pair. 2

The regions orient as shown in Figure

6.1b, but the angles deviate from 120

o because the

lone pair-bonding pair interaction is stronger than the bonding pair-bonding pair interaction, which closes the bond angle slightly. We conclude that molecules of this type are

bent

with

bond angles of ~120

o.

Green lobes represent lone pairs.

AX

E 2

: A has four electron groups, two bonding and two lone pairs. The regions orient as 2

shown in Figure 6.1c, but t

he angles deviate from the 109

o because the two lone pairs force

the bond angle to close from the predicte

d value. The shape of the molecule is

bent

with

bond angles of ~109

o (Figure 6.2c). Water is an AX

E 2

molecule, and the H-O-H bond 2

angle is actually 104

o. However, we will refer to the angle as simply ~109

o.

Table 6.1

Shapes of simple molecules

Molecule Shape Bond Angle AX

(^2)
linear
180
o^
AX
E bent ~120 2
o^
AX
E 2
bent ~109 2
o^
AX
(^3)
planar
120
o^
AX
: There are three identical bonding electron 3
groups around A that orient as shown in
Figure 6.1b. All four atoms lie in the same plane, so the molecule is said to be
planar
or
trigonal planar
to show that the three X atoms form a triangle (Figure 6.3a). The bond
angles are 120
o, which is not approximate because the three groups are identical.
AX
E pyramidal ~109 3
o^
AX
(^4)
tetrahedral 109
o^
Chapter 6 Molecular Structure & Bonding
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