Electronegativity may be defined as the tendency of an atom in a molecule to attract towards itself the shared pair of electrons.
It may be mentioned here that unlike ionization energy and electron affinity, electronegativity is not a measurable quantity. However, in order to compare the electronegativity values of different elements a number of numerical scales of electronegativity of elements have been proposed. The most common and widely use scale of electronegativity is the
Pauling’s scale of electronegativity is based on excess bond energies. Electronegativity of fluorine, the most electronegative element, is arbitrarily taken as 4.0 in this scale.
The main factors on which the electronegativity depends are effective nuclear charge and atomic radius.
• Greater the effective nuclear charge greater is the electronegativity.
• Smaller the atomic radius greater is the electronegativity.
The electronegativity of any given element is not const3nt but varies depending on the element to which it is bound.
In a period electronegativity increases in moving from left to right. This is due to the reason that nuclear charge increases whereas atomic radius decreases as we move from left to right in a period. Halogens have the highest value of electronegativity in their respective periods. The electronegativity values (on Pauling scale) of elements of second and third periods are listed in Table 6.12.
Table 6.12. Electronegativity Values of Elements of Second and Third Periods
|Elements of Second Period Li Be
Electronegativity 1.0 1.5
Elements of Third Period Na Mg
Electronegativity 0.9 1.2
| B C N O F
2.0 2.5 3.0 3.5 4.0
Al Si p s CI
1.5 1.8 2.1 2.5 3.0
In a group electronegativity decreases on moving down the group. This is due to the effect of increased atomic radius. for example, among halogens fluorine has the highest etectronegativity. In fact, fluorine has the highest value of etectronegativity among all the elements.
The electronegativity values (on Pauling scale) of group and group 17 elements are given in Table 6.13.
Table 6.13. Electronegativity Values of Group-1 and Group-17 Elements
Relationship between Electronegativity and Non-metallic or Metallic) Character of an Element
Non-metallic elements have strong tendency to gain electrons. Therefore, electronegativity is directly related to he non-metallic character of elements. We can also say that the electronegativlty is inversely related to the metallic character of elements. Thus, the increase in electronegativities .across a period is accompanied by an increase in non-metallic character (or decrease in metallic character) of elements. Similarly, the decrease in electronegativity down a group is accompanied by a decrease in non-metallic character or increase in metallic character) of elements.