Modern inorganic chemistry

50 STRUCTURE AND BONDING

Table 2.11

SOME ELECTRONEGAT1VITY VALUES (PAULING)

Li Be B C N O

2.1

F

4,0

Cl

3.0

1.0

Na

0.9

1.5

Mg

1.2

2.0

Al

1.5

2.5

Si

1.8

3.0

P

2.1

3.5

S

2.5

shared; a partial polarisation of the covalent bond is observed and
the two atoms exert an electrostatic attraction for each other. The
results of this attraction are a decrease in the bond length and an
increase in the bond strength from those values expected for a 'pure'
covalent bond*. There is in fact no sharp distinction between ionic
and covalent bonds and all 'degrees' of ionicity and covalency are
possible.

Resonance

Bonds with characteristics intermediate between ionic and covalent
can also be represented by, for example, two imaginary structures, I
and II both of which "contribute' to the true structure III. Consider
gaseous hydrogen chloride :

H— Cl

" d equal sharing unequal sharing

electrovalent covalent covalent

I II III

The strength of the bonding found in the actual structure III is
greater than that calculated for either of the imaginary structures
I and II. This has been explained on the theory of resonance based

• Pauling's electronegativity values are derived from the differences between 'pure
  covalent' and actual bond energies. Another simple measure of electronegativity is
  the sum of the ionisation energy and electron affinity, I + E. The more electro-
  negative elements have high values of / 4- £. Consider the alternative ionic forms of
  the diatomic species AB:. i.e. A+B~ or A"B +. To form the first in the gas phase
  requires an energy /A - £B ; to form the second requires an energy /B - £A Which-
  ever energy is the lesser will indicate the direction of electron transfer ; if A is more
  electronegative than B then we require that A~B+ is favoured and thus that
  fA - /B > JB - £A or /A + £A > /B + £B and on this basis the order of values of
  / 4- E indicates an electronegativity scale,