Chemistry - A Molecular Science

SF

requires a minimum of 4 shared pairs for th 4

e four S-F bonds, so the three shared pairs

determined above cannot be correct and th

e octet rule cannot be obeyed. The group

number of sulfur is 6 and its oxidation state in SF

is +4, so the number of lone pairs 4

around the sulfur atom is LP =

1 /^2

(6-4) = 1 lone pair. Thus, there are five electron groups

around the sulfur: four S-F bonds and one lone

pair. As shown in the margin, the five

groups adopt the trigonal bipyramidal structure with

the lone pair in the equatorial plane.

S

F F

F F

SF

4 Xe

F F

Example 6.4a XeF

2

Br

FFF

F^

Example 6.4b BrF

1-^4

Example 6.4 a) What is the shape of XeF

? 2

SP = ½ (24-22) = 1, which is insufficient for two Xe-F bonds, so Xe uses an expanded valence shell. The group number for Xe

is 8 and its oxidation state in XeF

is +2, so there 2

are ½(8-2) = 3 lone pairs around the xenon atom in XeF

. Two Xe-F bonds and three lone 2

pairs adopt a trigonal bipyramidal structure wi

th the lone pairs in the equatorial plane to

make XeF

a linear molecule as shown in the margin. 2

b) Describe the shape of the BrF

1- ion. 4

SP = ½ (40-36) = 2, which in insufficient for four Br-F bonds, so Br has an expanded valence shell. Bromine is a 7A nonmet

al and its oxidation state in BrF

1- 4

is 3, so it must

have ½(7-3) = 2 lone pairs. The six electron groups adopt an octahedral geometry with the lone pairs situated opposite to one another.

As shown in the margin, the four bromine

atoms are at the corners of a square with t

he bromine in the center. All five atoms are

coplanar, so the BrF

1- 4

ion is said to be square planar.

6.3

LARGER MOLECULES

Molecules are three-dimensional, and their bon

ds are seldom at right angles, but a Lewis

structure is typically a two-dimensional re

presentation in which bond angles are drawn at

180

o or 90

o. The structure of a molecule is inferred by applying what we have learned

about the shapes of molecules w

ith a single central atom to

each

atom in a larger

molecule: count the electron groups around an atom and then determine the geometry at that position. Most molecules twist and bend and are not rigid structures, so we cannot look at a Lewis structure of a large, flexible molecule and know its overall structure, but we can envision the possibilities. In this

section, we consider a few simple cases.

a

b

g

H^1

a

b

g

(a)

(b)

C

CO

HOH

H^3

2

Figure 6.7 Lewis structure (a) and ball-and-stick model (b) of acetic acid

Acetic acid

is HC

H 2

O 3

and has the Lewis structure shown in Figure 6.7a. The bond 2

angles can be determined by applying VSEPR to each of the atoms. There are four electron regions around the carbon at position 1, so it has a tetr

ahedral geometry with bond

Chapter 6 Molecular Structure & Bonding

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