Chemistry - A Molecular Science

Chapter 5 The Covalent Bond

Example 5.5

How many resonance forms can be drawn for SO

? 2

Use the method presented in Section

5.6 to determine the Lewis structure:

1. ER = 3 atoms

x8 electrons/atom = 24 electrons.

2. VE = 6 from sulfur + 6 from each oxygen = 6 + (2)(6) = 18 valence

electrons. The

Lewis structure must contain nine pairs of electrons.

3. SP = ½ (24 - 18) = 3 pairs must be shared. 4. The three shared pairs must be distributed as one double bond and one single bond,

but the double bond can be placed betw

een the sulfur and either oxygen.

Thus, SO

exists in two resonance forms, as shown in Figure 5.9. 2

OSO

OSO

Figure 5.9 Two resonance forms of SO

(^2)
Note that resonance is indicated by a line with arrows at both ends, and that curved arrows are used to show the movement of electron pairs. A curved arrow from a lone pair to a bond indicates that the lone pair becomes a bond, and a curved arrow from a bond to an atom indicates that the bonding pair becomes a lone pair on that atom.
When the different resonance forms are e
quivalent, each form contributes equally to
the bonding. The two resonance structures of SO
shown in Figure 5.9 are equivalent, so 2
the bonding is the average of both forms. Consequently, one of the bonding pairs is shared between the two equivalent bonds, and neither bond is a single or double bond. The two sulfur-oxygen bonds in sulfur dioxide are of equal length, both being shorter than a S-O single bond but longer than a S=O double bond. The bond order of each of the bonds affected by the resonance is equal to the
number of bonding pairs involved in those bonds
divided by the number of bonding regions
in which the bonds can be found. In SO
, there 2
are three bonding pairs spread over two sulfu
r-oxygen bonding regions, so the sulfur-
oxygen bond order in SO
is 2
(3 bonding pairs)
/(2 bonding regions)
= 1.5. Resonance in SO
(^2)
results because one of the bonding pairs is sp
read over both bonding regions rather than
localized in one. Note that curved arrows
are used to show a mechanism that converts
between the two resonance forms. Curved arro
ws from a lone pair to a bond indicate that
the lone pair becomes a bonding pair, while
a curved arrow from a bond to an atom
indicates that the bond becomes a lone pair.
5.8
FORMAL CHARGE AND OXIDATION STATE Bonding can result in an uneven distribution of the bonding electrons to produce electrically charged regions within a molecule

. Chemists use these regions of charge to

explain both chemical and physical properties of the molecules, so assigning charge to the atoms in a molecule is an important task in

chemistry. Charge is assigned to the atoms by

assigning

each of the valence electrons in a molecule to one of the atoms. The nonbonding

electrons are readily assigned to their atoms, but in order to assign the bonding electrons,

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