Chemistry, Third edition

RESONANCE STRUCTURES

same length; this bond length is shorter than would be expected for a C–O bond, but

longer than that for C=O. The ‘real’ structure of the carbonate ion is a mixture of the

three structures that we have already written and can be represented by:

This ‘mixing’ of structures is called resonanceand the resulting structure, such as

the one written above, is called a resonance hybrid. This idea is similar to the rela-

tionship between a donkey, a mule and a horse. The mule is the ‘resonance hybrid’ –

it is neither a donkey nor a horse but has features of both of them.

Box 4.7 gives some historical information on scientists who have contributed to

bonding theory.

61

BOX 4.7

G. N. Lewis and Linus Pauling

Gilbert Lewis (1875–1946) was the first

American to win the Nobel prize. This

was awarded in 1914. Referring to

covalent bonding, he wrote ‘an electron

may form part of the shell of two

different atoms and cannot be said to

belong to either one exclusively’. He was

not without his critics, however. It was

argued that this statement was the

same as saying that if a couple have

two dollars in a joint bank account and

six dollars each besides (total 14

dollars), they both have eight dollars

apiece (total 16 dollars). Lewis

structures, however, still explain the

properties of a great deal of covalent

compounds today – the bank account

analogy does not hold true in the world

of chemistry!

Linus Pauling (1901–1994), who

proposed electronegativity values, wrote

The Nature of the Chemical Bondin

1939. It is still regarded as a classic
      chemistry textbook. He received not
      one, but twoNobel prizes: one for
      chemistry in 1954 and one for peace in


1940. The peace prize was awarded for
      his campaign against the testing of
      nuclear weapons. He did not take the
      courses required to graduate from high
      school and only received his diploma
      afterhe had gained two Nobel prizes! He
      advocated very large doses of vitamin C
      as a preventative treatment for colds
      and cancer. This treatment is still very
      controversial, but it cannot be denied
      that he lived to a ripe old age and was
      still directing scientific research at the
      end of his life.

Gilbert Newton Lewis, an American

chemist. His theories led to a clearer

understanding of the concept of

valency.

Early sketches of his ideas, by G. N.

Lewis. He eventually became professor

of chemistry at the University of

California.

Linus Pauling, an American chemist

and twice Nobel Prize winner. His book

The Nature of the Chemical Bondis

regarded as a classic by chemists.

Resonance

Structures

(i)iThere are two possible

structural formulae that

might represent NO 2 .

Draw both of these.

(ii)Draw the best

representation of the

structural formula of the

NO 2 ion; it is a

resonance hybrid.

Exercise 4L