Electronegativity Scales

ELECTRONEGATIVITY SCALES

Electronegativity scale is an arbitary scale. They are based on various types of experimental data like bond energy, dipole moment, ionisation potential and electron affinity. Most commonly used scales are

1. Pauling's scale

2. Mulliken's scale

3. Sanderson's scale

4. Alfred and Rochow's scale

1. Pauling's scale (1932)

This scale is based on an empirical relation between the energy of a bond and the electronegativities of bonded atoms.

Consider a bond A-B between two dissimilar atoms A and B of a molecule AB. Let the bond energies of A-A, B-B and A-B bonds be represented as EA-A, EB-B and EA-B respectively. It may be seen that the bond dissociation energy of A-B is almost higher than the geometric mean of the bond dissociation energies of A-A and B-B bonds i.e.,

E _A-B  > root ( E _A-A  × E _B-B )

Their difference (D) is related to the difference in the electronegativities of  A and B according to the following equation

D = E _A-B  -  root ( E _A-A  × E _B-B )

 = (X_A-X_B)²

^OR

0.208 root(D) = X_A - X_B

Here, XA and XB are the electronegativities of A and B respectively. The factor 0.208 arises from the conversion of Kcals to electron volt. Considering arbitarily the electronegativity of hydrogen to be 2.1, Pauling calculated electronegativities of other elements with the help of this equation.

Disadvantage of Pauling scale

The disadvantage of Pauling's scale is that bond energies are not known

with any degree of accuracy for many solid elements.

Problem

Calculate the electronegativity of chlorine from the following data

EH-H = 104 K cal mol-1 ; ECl-Cl = 36 K cal mol-1

EH-Cl = 134 K cal mol-1

According to Pauling's equation

D = EHCl -  root( EH-H  x ECl-Cl    )

= 72.82

0.208 root(D) = XCl - XH

XCl - 2.1 = 1.77

XCl  = 3.87

2. Mulliken's Scale

In 1934, Mulliken suggested an alternative approach to electronegativity based on ionization energy and electron affinity of an atom. According to this method electronegativity could be regarded as the average of the ionization energy and electron affinity of an atom

Electronegativity = (I.E. + E.A) / 2

Mulliken used ionisation energy and electron affinity values measured in electron volts and values were found to be 2.8 times higher than Pauling values.

The values of ionisation energy and electron affinity are measured in kJ mol-1 and 1eV =96.48 kJ mol-1.

Therefore the commonly accepted Pauling values are more nearly obtained By

Electronegativity  = (I.E. + E.A) / (2× 2.8× 96.48)  = (I.E. + E.A) / 50

This method has an ordinary theoretical basis and also has advantage that different values can be obtained for different oxidation states of the same element.

Disadvantage

Although Mulliken's scale is less empirical than Pauling Scale, yet it suffers from a serious disadvantage that electron affinities with the exception of a few elements are not reliably known.

Problem

Calculate the electronegativity values of fluorine and chlorine on Mulliken's scale, given that (Ionisation potential) F = 17.4 eV/atom (Electron affinity) F = 3.62 ev/atom, (IP)Cl = 13.0 ev/atom and (EA)Cl = 4.0 ev

Electronegativity of fluorine = (IP)F + (EA)F     /  2x2.8   = 3.75

Electronegativity of chlorine = (IP)Cl + (EA)Cl  / 2x2.8 = 3.03