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 )
= (XA-XB)2
OR
0.208 root(D) = XA - XB
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
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