Modern inorganic chemistry

H

0.037

Si

0.117

C

0.078

P

0.110

N

0.070

s

0.104

0

0.066

Cl

0.099

STRUCTURE AND BONDING 49

F

0.064

Deductions of bond lengths for any unknown can be made by adding

bond radii, but these theoretical values often differ from the experi-

mental values; the greatest deviations occur when elements of widely

different electronegativities are joined together.

ELECTRONEGATIVITY

If two like atoms form a covalent bond by sharing an electron pair,

for example

xF*XX XX Fx

it is clear that the pair will be shared equally. For any two unlike

atoms, the sharing is always unequal and depending on the nature

of the two atoms (A and B say) we can have two extreme possibilities

or A :B i.e. A+ B"

A + B->A : B ^

equal

sharing Xor A; g ie A- B +

and an ionic bond is formed. There are many compounds which lie

between truly covalent (equal sharing) and truly ionic. The bond

between two atoms A and B is likely to be ionic rather than covalent

(with A forming a positive ion and B a negative ion) if:

1. A and B have small charges


2. A is large


3. B is small

Tables 2.1, 2.2, 2.3 and 2.4 give data for atomic radii, ionisation
energies and electron affinities which allow these rough rules to be
justified.
Pauling and others have attempted to define an 'electronegativity
scale' by which the inequality of sharing might be assessed. Some of
Pauling's electronegativity values are shown in Table 2.11. The
greater the differences in the electronegativities of the two elements
joined by a covalent bond, the less equally the electrons are