Chemistry - A Molecular Science

Chapter 5 The Covalent Bond

Example 5.10

Draw the Lewis structure of SO

, indicate all nonzero formal charges, and 3

determine the oxidation state of each atom.

OS

O

OO

O S

OOS

O

O

+2

+2

+2

Example 5.10 Resonance structures of SO

(^3)

1. ER = (4 atoms)(8 electrons/atom) = 32 electrons. 2. VE = (1)(6) from sulfur + (3)(6) from

oxygen = 24 valence electrons or 12 pairs.

3. SP =

1 /^2

(32 - 24) = 4 pairs must be shared in the three sulfur-oxygen bonds.

The Lewis structure of SO

shows 24 electrons in four shared pairs and eight lone pairs. 3

The three sulfur-oxygen bonds share the double bond because the three resonance structures are identical. The act

ual structure is a combination

of all three structures shown

in the margin, and the S-O bond order is

4 /^3

(four shared pairs in three S-O bonds). Note

that Lewis structures always include all equivalent resonance forms. The nonzero formal charges are acceptable here because (1) no Lewis structure can be drawn for SO

that 3

has all zero formal charges, and (2) the negative formal charge resides on the more electronegative oxygen atoms. The formal charges on the atoms are FC

= 6 VE - [0 NB + S

1 /^2

(8 BE)] = +2

FC

= 6 VE - [4 NB + O

1 /^2

(4 BE)] = 0

(double bonded O)

FC

= 6 VE - [6 NB + O

1 /^2

(2 BE)] = -1

(each single bonded O)

The sum of the formal charges is +2 + 0 +2(-1) = 0, as it must be for a molecule. Oxygen is more electronegative than sulf

ur, so the oxidation states are:

OX

= 6 VE - [0 NB + (0)(8 BE)] = +6 S

OX

= 6 VE - [4 NB + (1)(4 BE)] O

= -2 (double bonded O)

OX

= 6 VE - [6 NB + (1)(2 BE)] = -2 (each single bonded O) O

Note that the sum of the oxidation

states is +6 + (-2) +2(-2) = 0.

Example 5.11

Draw the Lewis structure of SO

2-, indicate all nonzero formal charges, and 3

determine the oxidation state of each atom. 1. ER = 4

x8 = 32 electrons

2. VE = (1)(6) from S + (3)(6) from O + 2 for the -2 charge = 26 valence electrons 3. SP =

1 /^2

(32 - 26) = 3 pairs must be shared.

All three SO bonds are single bonds because

there are only three shared pairs to bond

three oxygens to the sulfur. The three shared

pairs give the sulfur six electrons, so it

needs 1 lone pair to complete its octet. Each oxygen must have three lone pairs to obtain an octet. Note that 26 electrons are sh

own in the structure in the margin.

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