Chemistry - A Molecular Science

Chapter 6 Molecular Structure & Bonding

AX

E: A has four electron regions, which or 3

ient as shown in Figure 6.1c. Unlike AX

, the 3

central atom is not in the plane of the three X atoms, so th

e four atoms form a pyramid and

the molecule is said to be

pyramidal

or

trigonal pyramidal

to indicate that the base of the

pyramid has three corners (Figure 6.3b). The bond angles are ~109

o because the four

regions are not identical.

NH

is an AX 3

E molecule, and the H-N-H bond angle is 107 3

o,

which is greater than the H-O-

H bond angle in water because the single lone pair in NH

(^3)
does not close the angle as much as the two lone pairs in H
O. 2
dashed wedge
solid wedge
line
Figure 6.4 Line-wedge-dash representation
HN
H
H
(a)
(b)
(c)
(d)
N
H
HH
Figure 6.5 Representations of ammonia (NH
) 3
(a) Lewis structure (b) Line-wedge-dash drawing (c) Ball-and-stick (d) Space-filling model Note that the ball-and-stick and space filling models do not show the lone pairs. Ball-and-stick and space-filling models of CO
(^2)
AX
: A has four identical bonding regions, which 4
orient as shown in Figure 6.1c. The four X
atoms are at the corners of a tetrahedr
on and the molecule is described as tetrahedral
with
109
o bond angles (Figure 6.3c).
MOLECULAR REPRESENTATIONS In Chapter 5, we represented molecules as their Lewis structures, and we will continue to do so throughout this text. However, we w
ill also use three other representations: line-
wedge-dash, ball-and-stick, and space-filling. In a
line-wedge-dash
representation, such
as the one shown in Figure 6.4, lines represent bonds between two atoms that lie in the plane of the paper, solid wedges are used to
show bonds to atoms that lie in front of the
plane of the paper, and dashed wedges indicat
e bonds to atoms that lie behind the plane of
the paper. In a
ball-and-stick
model, atoms are represented as spheres and bonds as
cylinders. Ball and stick models are the best representation to use when discussing molecular structure because the bond angles are easy to see. A
space-filling
model, which
shows atoms as spheres and bonds as the penetr
ation of two spheres into one another,
gives the truest picture of the molecule. Th
e relative diameters of the spheres represent the
relative sizes of the atoms. Lone pairs were
shown in the ball-and-stick models in Figure
6.3 to emphasize their role in determining structure, but lone pairs are not normally included in either the ball-and-stick or space
-filling representations. The various ways in
which we will represent ammonia, an AX
E molecule, are shown in Figure 6.5. 3
Example 6.1
Use the Lewis structures of CO
, SO 2
, and H 2
O determined in Chapter 5 to predict 2
their shapes and to estimate their bond angles. CO
: 2
The Lewis structure of CO
is 2
OC
O
There are no lone pairs on C (an AX
molecule), so the three atoms lie on a line, and the 2
shape is described as linear (bond angle = 180
o). The ball-and-stick and space-filling
models of CO
are presented in the margin. 2
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