The Foundations of Chemistry

Halogen atoms can appear in place of hydrogen atoms in many organic compounds,
because hydrogen and halogen atoms each need one more electron to attain noble gas
configurations. An example is chloroform, CHCl 3. Alcohols contain the group CXOXH;
the simplest alcohol is methanol, CH 3 OH. An organic compound that contains a carbon–
oxygen double bond is formaldehyde, H 2 CO.

H

H

H CC O H

Cl

H

Cl C Cl

O

HH

C

chloroform, CHCl 3 methanol, CH 3 OH formaldehyde, H 2 CO

7-6 Resonance 287

RESONANCE

In addition to the Lewis formula shown in Example 7-3, two other Lewis formulas with
the same skeleton for the CO 32 ion are equally acceptable. In these formulas, the double
bond could be between the carbon atom and either of the other two oxygen atoms.

A molecule or polyatomic ion for which two or more Lewis formulas with the same
arrangements of atoms can be drawn to describe the bonding is said to exhibit resonance.
The three structures above are resonance structuresof the carbonate ion. The rela-
tionship among them is indicated by the double-headed arrows, mn. This symbol does not
meanthat the ion flips back and forth among these three structures. The true structure
can be described as an average, or hybrid, of the three.

O

OO O

C

2 

O

OO O

C

2  2 

O

C

7-6

Problem-Solving Tip:Some Guidelines for Drawing Lewis Formulas

The following guidelines might help you draw Lewis formulas.

1.In most of their covalent compounds, the representative elements follow the octet

rule, except that hydrogen always shares only two electrons.

2.Carbon always forms four bonds. This can be accomplished as:

a.four single bonds

b.two double bonds

c.two single bonds and one double bond or

d.one single bond and one triple bond

3.Hydrogen forms only one bond to another element; thus hydrogen can never be a

central atom.

4.In neutral (uncharged) species, nitrogen forms three bonds, and oxygen forms two

bonds.

5.Nonmetals can form single, double, or triple bonds, but never quadruple bonds.

6.Carbon forms double or triple bonds to C, N, O, or S atoms; oxygen can form

double bonds with many other elements.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 9.6, Resonance Structures.

C, N, and O often form double and

triple bonds. S can form double bonds

with C, N, and O.