Chemistry - A Molecular Science

Example 5.7

OCO

OCO

AB

• 
  

+

Figure 5.12 Structure A is the prefer

red resonance form of CO

because there 2

is no charge separation. The non-zero formal charges on the oxygen atoms in structure B make it less favorable.

O C

HF

O CH

F

O CF

H

Example 5.7b

a) Which of the two resonance forms of CO

shown in Example 5.4 is preferred? 2

Refer to Figure 5.12 for the Lewis structures of the two resonance forms. Structure A: Each oxygen has four nonbondi

ng electrons and one-half of the four bonding

electrons, for a total of six valence electr

ons. Thus, both oxygen atoms have zero formal

charge. Alternatively, each oxygen has zero

formal charge because each is involved in

two bonds. The carbon has one-half of the eight

bonding electrons for a total of four

valence electrons, which is the same as th

e free atom. Consequently, the carbon atom

also has a zero formal charge. Alternatively, each carbon has zero formal charge because each is involved in four bonds. Structure B: Neither oxygen has two bond

s, so they each have nonzero formal charge.

The triple bonded oxygen has two nonbonding electrons and one-half of six bonding electrons, for a total of five valence electr

ons, one less than the free atom. Thus, this

oxygen carries a +1 formal charge. The single bonded oxygen contains six nonbonding and one-half of two bonding electrons, for a

total of seven valence electrons, one more

than the free atom. It, therefore, carries a -1

formal charge. The carbon has one-half of

eight bonding electrons and therefore a zero formal charge. Structure A is preferred because it has no charge separation.

b) The Lewis structure of FCHO requires four shared pairs. Use formal charge to

decide which of the structures in the margin is preferred. Structure A

:

FC

= 7-6-1 = 0; F

FC

= 6-4-2= 0; O

FC

= 4-0-4 = 0 C

Structure B

:

FC

= 7-4-2 = +1; F

FC

= 6-6-1= -1; O

FC

= 4-0-4 = 0C

Structure C

:

FC

= 7-4-2 = +1; F

FC

= 6-4-2= 0; O

FC

= 4-2-3 = -1 C

Structures B and C both place two bonds to

F, which puts positive formal charge on the

most electronegative atom. There is only a single

shared pair to F in Structure A, so it has

no formal charge. Structure A is the preferr

ed structure because it contains no formal

charge – only one bond to a terminal halogen and four bonds to carbon. Example 5.7 demonstrates that

placing more than one bonding pair to a halogen

should be avoided

because it places positive formal

on these electronegative atoms.

Placing positive formal charge on a halogen is

unavoidable in compounds or ions in which

it is the central atom. However, the other

atoms are highly electronegative in these cases,

so the positive formal charge is more reasonable. For example, the formal charge on Cl in ClO

1- 4

is +3. Recall from Chapter 4, that the oxidation state of Cl in ClO

1- 4

is +7, so a

formal charge of +3 is still much less than the charge the chlorine atom would have if the bonds were ionic.

Chapter 5 The Covalent Bond

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