Chemistry - A Molecular Science

Chapter 6 Molecular Structure & Bonding

Example 6.3

What is the shape of the sulfite ion, SO

2-? 3

The Lewis structure shows four electron regi

ons around the sulfur atom (three S-O bonds

and one lone pair). As a result, the sulfur at

om becomes the apex of a trigonal pyramid.

The sulfite ion is trigonal pyramidal, and the O-S-O bond angle is ~109

o.

Ball-and-stick and space-filling models of SO

2-^3

PF

5

SF

6

(a) trigonal bipyramidal

(b) octahedral

Figure 6.6 Molecules with five and six electron groups a) The green spheres occupy equatorial positions, while the

blue spheres are in the axial positions. Lone pairs occupy equatorial positions.

b) All of the positions of

an octahedron are identical.

OS

O

O

S

O

OO

~109

o

6.2

CENTRAL ATOMS WITH EXPANDED VALENCE SHELLS The octet rule applies rigidly only to C, N, O and F, and even nitrogen violates it occasionally because it has an odd number of electrons in some of its molecules (

e.g

.,

NO). This may seem quite restrictive until you realize that C is the basis of organic chemistry, and the number of compounds that can be made using only C, N, H and O is limitless. Atoms after the second period often ha

ve more than eight valence electrons. An

atom with more than an octet of valence electrons is said to have an

expanded valence

shell

. While our discussion of atoms with expande

d valence shells is postponed to Chapter

14, Inorganic Chemistry, we show the two most common structures adopted by such molecules in Figure 6.6. Five groups around a central atom adopt a

trigonal bipyramidal

structure (Figure 6.6a), which contains two

distinctly different types of positions: two

positions are

axial

(blue spheres) and three are

equatorial

(green spheres). Interactions

with other groups are less in the equatorial

positions, so lone pairs are always in the

equatorial plane. Six groups assume an

octahedral

structure (Figure 6.6b). All six

positions of an octahedron are identical, so lone

pairs can be placed in any one. However,

two lone pairs are always situated opposite

to one another. The number of lone pairs

around a central atom (LP) can be determ

ined from its group number and its oxidation

state as follows:

LP =

1 /^2

[Group Number – Oxidation State]

Eq. 6.1

Atoms with expanded valence shells can be identified because the predicted number of

shared pairs is always too small to accommoda

te all of the bonds. For example, if we were

to attempt to draw the Lewis structure of SF

, we would determine the following: 4

ER = 5(8) = 40 electrons; VE = 6 + 4(7) = 34 electrons; SP =

1 /^2

(40 - 34) = 3 shared pairs

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