Ask A Scientist© Chemistry Archive

name         Alma S.
status       educator
age          20s
Question -   How is the electronegativity determinated experimentally?
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Electronegativity is not determined experimentally. It is
am empirical number used to account for a variety of chemical
phenomena by assigning electron-withdrawing power of atomic
types on a relative, average basis.
One exception (which no one uses) is a formula that Mullekin
cooked up to explain trends in electronegativity based on
atomic properties. It involves averaging the magnitudes of
the atomic ionization energy and the atomic electron affinity.
However, electron affinities have not been measured for all
the elements and so this formula is of at best very limited
utility.

best,
prof. topper
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Electronegativity is an empirical measure of the tendency of an atom to
attract electrons. There are several schemes that have been proposed to
quantify this empirical tendency. One is due to Linus Pauling. Another is
due to Mulliken. The argument goes something like this:

For two atoms, A and B, the energy difference,E, between A+B- and A-B+ is
given by:
                 E(A+B-) - E(A-B+) = [IP(A) - EA(A)] - [IP(B) - EA(B)]
                                                 = [IP(A) + EA(A)] - [IP(B)
+EA(B)]

where IP is the ionization potential of the specified atom, A or B, and EA
is the electron affinity of the specified atom. The ionization potential IP
is known for a large number of atoms and molecular fragments from mass
spectrometry. The electron affinity data are more sparce. It can be measured
spectroscopically from the dissociation limit of the ionic species A-, or
can be calculated from quantum mechanics for simpler atoms.

Mulliken proposed that the electronegativity DIFFERENCE X(A) and X(B)between
two atoms A and B is:

                 X(A) - X(B) = 1/2*[(IP(A) + EA(A)) - (IP(B) + EA(B))]

or, rearranging to assign a value to A and B individually:

             X(A) = 1/2*[IP(A) +EA(A)]    and    X(B) = 1/2*[IP(B) + EA(B)]

Because electronegativity is an empirical quantity historically they have
been "fixed up" with various corrections.  As a result, when using
electronegativities to make some chemical argument it is important to make
sure the values used come from a self-consistent set of values.

Vince Calder
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Electronegativity is a derived quantity, not a true quantity.  The earliest
definition of electronegativity was put forth by Mulliken, as the average of
the atom's ionization energy and its electron affinity.  The ionization
energy is the energy required to remove one electron from the neutral atom;
the electron effinity is the energy required to remove one electron from the
-1 ion. Both of these quantities can be measured in several ways;  the
conceptually simplest is by a technique known as photoelectron spectroscopy,
in which light provides the energy to eject an electron from the atom or
ion.  The hard part, experimentally, is preparing the sample .

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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