The Science Book

(Elle) #1

258 LINUS PAULING


with the structure of carbon
dioxide (CO 2 ), where the sp hybrids
each form a sigma bond with the
oxygen, and a second pi bond is
formed by the remaining two
unhybridized orbitals.


A new structure of benzene
The structure of benzene, C 6 H 6 , had
worried August Kekulé when he
first proposed that it was a ring,
more than 60 years earlier. He
eventually suggested that the
carbon atoms must be connected
with alternate single and double
bonds, and that the molecule
oscillated between the two
equivalent structures (p.164).
Pauling’s alternative solution
was elegant. He said that the
carbon atoms were all sp^2
hybridized, so that the bonds
between them and the hydrogen
atoms all lie in the same xy plane
and form an angle of 120° with each
other. Each carbon atom has one
remaining electron in a pz orbital.
These electrons combine to form
a bond connecting all six carbon
atoms. This is a pi bond, and, in
it, the electrons remain above and
below the ring, and away from the
carbon nuclei (see right).


Ionic bonding
Methane and ethylene are gases
at room temperature. Benzene and
many other organic compounds
based on carbon are liquids. They
have small, lightweight molecules
that can easily move around in the
gas or liquid state. Salts such as
calcium carbonate and potassium
nitrate, by contrast, are almost
invariably solids, and melt only
at high temperatures. And yet a
unit of sodium chloride (NaCl) has
a molecular weight of 62, while
benzene has a molecular weight
of 78. The difference in their
behavior is explained not by


their weight, but by their structure.
Benzene is held together in single
molecules by covalent bonds
between the atoms; that is,
each bond comprises one pair
of electrons shared between
two specific atoms.
Sodium chloride has quite
different properties. The silvery
metal sodium burns energetically
in the greenish gas chlorine to
produce the white solid sodium
chloride. The sodium atom has a
stable complete shell of electrons
around the nucleus, plus one spare

electron outside that. The chlorine
atom is one electron short of a
stable complete shell. When they
react, an electron is transferred
from the sodium atom to the
chlorine atom, and both acquire
stable complete shells of electrons,
but now the sodium has become a
sodium ion Na+, and chlorine has
become the chloride ion Cl- (see
above). They have no spare
electrons to form covalent bonds,
but the ions are now charged: the
sodium atom has lost a negatively
charged electron so now has a

In sodium chloride, an electron in the sodium
atom moves into a chlorine atom to form two
charged, stable ions. The ions are held together
by electrostatic attraction to form a stable lattice.

Ionic bonding

In a benzene ring, the carbon atoms are bonded
to each other and a hydrogen atom by sp^2 hybridized
orbitals. The rings are bonded to each other by
a nonlocalized pi-bond formed from the six pz orbitals.

Sodium ion
Na+

Chloride ion
Cl-

H

H

H

H

sp^2 hybridized orbitals 6 pz orbitals Pi bond

H

H
H

H

H
H

H

H

H

H

H

H

H

H

Benzene ring

Lattice
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