Chemistry Reference
In-Depth Information
electronegativity is a qualitative concept that is not amenable to direct measure-
ment. Electronegativity was evaluated by various indirect methods that resulted in
several electronegativity scales named after their originator. Experience has shown
that they lead to scales that have similar values, although different methods were used.
The Pauling scale, named after the chemist Linus Pauling, is based on thermody-
namic data associated with bond energy, the energy that must be provided to break a
bond between two atoms. Considering formation of a polar molecule AB from non-
polar molecules A
2
and B
2
, the bond A-B contains a supplementary energy relative
to the average of energies A-A and B-B. Pauling considers this energy as an ionic
contribution to the covalent bond, correlated with the electronegativity difference
between atoms A and B:
1
2
(
)
E
=
EE
+
+∆
(3.1)
AA
−
BB
−
AB
−
where Δ is a bonding extra-ionic energy.
If χ
A
and χ
B
are the electronegativities of the atoms A and B, Pauling proposed
the following formula:
χ−χ= ∆
0.102
with Δ calculated in kJ⋅mol
−1
.
(3.2)
A
B
The scale is chosen so that the relative electronegativity of hydrogen (χ
H
) would be
2.1. With the exception of the noble gases, dimensionless values of relative electro-
negativity are calculated for all chemical elements relative to the hydrogen value.
Variation of the electronegativity (Pauling scale) is dependent on the atomic number
Z for the elements from the first three periods (Figure 3.1).
Electronegativity shows the same variation as ionization potential; 1 that is, it
increases in a period from left to right and decreases from top to bottom in a group.
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
13
57911 13 15 17 19
Z
21 23 25 27 29 31 33 35
FIGURE 3.1
Variation of electronegativity for the elements from the first three periods
(Pauling scale).
