A PARADIGM SHIFT 259
positive overall charge; the chlorine
atom has gained an electron and has
a negative charge. The ions are held
together by electrostatic attraction,
plus to minus—a strong bond.
Sodium chloride was the first
compound to be analyzed by X-ray
crystallography. It was found that
in reality there is no such thing
as a molecule of NaCl. The
structure comprises an infinite
array of alternating sodium and
chloride ions. Each sodium ion is
surrounded by six chloride ions,
and each chloride is surrounded
by six sodiums. Many other salts
have similar structures: infinite
lattices of one type of ion with
different ions filling all the gaps.
Electronegativity
Pauling explained ionic bonding
in compounds such as sodium
chloride, which is purely ionic,
and also compounds in which
the bonding is neither purely
ionic nor purely covalent but
somewhere in between. This
work led him to develop the
concept of electronegativity, which
to some extent echoed the list of
metals in decreasing order of
electropositivity first introduced by
Alessandro Volta in 1800. Pauling
discovered that the covalent bond
formed between atoms of two
different elements (e.g. C–O) is
stronger than might be expected
from the average of the strengths
of C–C bonds and O–O bonds. He
thought that there must be some
electrical factor that strengthened
the bond, and set out to calculate
values for this factor. The scale is
now known as the Pauling scale.
The electronegativity of an
element (strictly speaking in
a particular compound) is a
measure of how strongly an
atom of the element attracts
electrons toward itself. The
most electronegative element is
fluorine; the least electronegative
(or the most electropositive)
of the well-known elements is
cesium. In the compound cesium
fluoride, each fluorine atom pulls
an electron entirely away from
a cesium atom, resulting in an
ionic compound Cs+F-.
In a covalent compound such
as water (H 2 O), there are no
ions, but oxygen is much more
electronegative than hydrogen,
and the result is that the water
molecule is polar, with a small
negative charge on the oxygen
atom and a small positive charge
on the hydrogen atoms. The
charges make the water molecules
stick together strongly. This
explains why water has so much
surface tension and such a high
boiling point.
Pauling first proposed a scale
of electronegativity in 1932, and
he and others developed it further
in subsequent years. For his
work elucidating the nature of the
chemical bond, he won the Nobel
Prize in Chemistry in 1954. ■There is no area of the world
that should not be investigated
by scientists. There will
always remain some questions
that have not been answered.
In general, these are the
questions that have not yet
been posed.
Linus Pauling
Linus Pauling
Linus Carl Pauling was born
in Portland, Oregon, US. He
first heard about quantum
mechanics while still in
Oregon, and won a scholarship
to study the subject under
some of the world experts in
Europe in 1926. He returned
to become assistant professor
at California Institute of
Technology, where he
remained for most of his life.
Pauling took great interest
in biological molecules,
and he discovered that
sickle-cell anemia is a
molecular disease. He was
also a peace campaigner, and
was awarded the Nobel Peace
Prize in 1963 for attempting
to mediate between the
US and Vietnam.
In later life, his reputation
was damaged as a result of
his enthusiasm for alternative
medicine. He championed the
use of high-dose vitamin C as
a defense against the common
cold, a treatment that has
subsequently been shown
to be ineffective.Key work1939 The Nature of
the Chemical Bond and the
Structure of Molecules
and Crystals