8.6 Appendix: Electronegativity (χ)
225
always found larger than the average. Pauling attributed this increase to the difference of electronegativity of the two atoms. He originally defined the difference in
electronegativity between the atoms A and B as
|χ A − χ B | =
1
√
eV
E(AB) −
E(AA) + E(BB)
2
(8.20)
where the dissociation energies E are expressed in eV.
Latter, he replaced the arithmetic mean by a geometric mean, which gives better
results. The new definition is
|χ A − χ B | = 0.102
E(AB) −
E(AA) × E(BB)
(8.21)
the dissociation energies E being expressed in kJ mol
−1 .
This equation only gives differences; an origin is needed. It was assumed that the
most electronegative element, fluorine, had the value 3.98.
8.6.3 Allred-Rochow Scale (Allred and Rochow 1958)
The electronegativity is evaluated from the Coulombic force of attraction between
the effective nuclear charge and that of an outer electron. Its expression is
χ = 3590
Z eff
r 2
cov
+ 0.744
(8.22)
The effective nuclear charge, Z eff , can be estimated using Slater’s rules and r cov
is the covalent radius expressed in pm.
8.6.4 Other Scales
Mulliken (1934) defined the electronegativity as the average of the energy required
to remove an electron from an atom, i.e., the ionization energy E i , and the energy
released by gain of one electron, i.e., the electron affinity E ea . Its expression is
χ =
E i + E ea
2
(8.23)
This electronegativity has the same unit as E i + E ca , usually eV. However, it is
usual to transform these absolute values into values comparable to the Pauling values,
χ = 1.97 × 10
−3
(E i + E ca ) + 0.19
(8.24)
225
always found larger than the average. Pauling attributed this increase to the difference of electronegativity of the two atoms. He originally defined the difference in
electronegativity between the atoms A and B as
|χ A − χ B | =
1
√
eV
E(AB) −
E(AA) + E(BB)
2
(8.20)
where the dissociation energies E are expressed in eV.
Latter, he replaced the arithmetic mean by a geometric mean, which gives better
results. The new definition is
|χ A − χ B | = 0.102
E(AB) −
E(AA) × E(BB)
(8.21)
the dissociation energies E being expressed in kJ mol
−1 .
This equation only gives differences; an origin is needed. It was assumed that the
most electronegative element, fluorine, had the value 3.98.
8.6.3 Allred-Rochow Scale (Allred and Rochow 1958)
The electronegativity is evaluated from the Coulombic force of attraction between
the effective nuclear charge and that of an outer electron. Its expression is
χ = 3590
Z eff
r 2
cov
+ 0.744
(8.22)
The effective nuclear charge, Z eff , can be estimated using Slater’s rules and r cov
is the covalent radius expressed in pm.
8.6.4 Other Scales
Mulliken (1934) defined the electronegativity as the average of the energy required
to remove an electron from an atom, i.e., the ionization energy E i , and the energy
released by gain of one electron, i.e., the electron affinity E ea . Its expression is
χ =
E i + E ea
2
(8.23)
This electronegativity has the same unit as E i + E ca , usually eV. However, it is
usual to transform these absolute values into values comparable to the Pauling values,
χ = 1.97 × 10
−3
(E i + E ca ) + 0.19
(8.24)