Chemistry Reference
In-Depth Information
It follows that fluorine is the most electronegative of the elements and cesium is the
least electronegative. Metallic elements are characterized by low electronegativities,
generally lower than 2.1. The exceptions are gallium and germanium, which have
higher electronegativities than aluminum and silicon, respectively, because of the
d-block contraction.
Allred and Rochow suggested a scale of electronegativity (Allred and Rochow
1958) based upon the electrostatic force of attraction between the nucleus and the
valence electrons:
2
eZ
eff
4r
(3.3)
F
=
2
πε
0
where
•
r is the distance between the electron and the nucleus (covalent radius),
•
e is the charge on an electron,
•
Z
eff
is the effective nuclear charge at the electron due to the nucleus and its
surrounding electrons (calculated from Slater's rules), and
•
ε
o
is vacuum permittivity (ε
0
≈ 8.854 × 10
−12
F·m
−1
).
The larger the effective nuclear charge (Z
eff
) and the smaller the atomic radius, the
more electronegative the element will be.
The quantity Z
eff
/r
2
correlates well with Pauling electronegativities and the two
scales can be made to coincide by expressing the Allred-Rochow electronegativity as
χ
= 0.744 + 0.359Z
eff
/r
2
(3.4)
where r is the covalent radius expressed in picometers (1 pm = 10
−12
m).
The Mulliken scale considers electronegativity as a measure of the tendency of an
atom to attract electrons (Mulliken 1934). Electronegativity is calculated as the arith-
metic mean between the first ionization energy (IE) and the electron affinity (EA),
1
2
(
)
.
(3.5)
χ= +
EI
EA
M
Because this definition is not dependent on an arbitrary relative scale, it has also
been termed
absolute electronegativity
, with the units of kilojoules per mole or elec-
tron volts. If an element has low values of both EI and EA, as is the case of most
metals, electronegativity is also low.
It is more usual to use a linear transformation of these absolute values into the
Pauling scale (Bratch 1988),
χ= χ− ±
1.35
1.37
0.14
.
(3.6)
P
M
The use of Mulliken electronegativity is limited by the determination of the electron
affinity, which is difficult to carry out for some elements.
