COMMENTING THE PHYSICAL MEANING OF USUAL ELECTRONEGATIVITY SCALES

Electronegativity can be certainly considered as a cornerstone concept in Chemistry. It allows simple and useful qualitative descriptions of the electron density distribution in bonds. As for other chemical basic concepts, lots of definitions can be found for it. All of them point to the same idea, but their real physical meaning is not the same or is, even, ambiguous. The IUPAC Gold Book defines electronegativity as “the power of an atom to attract electrons to itself”, but does not say anything about how this power has to be measured. Popular text books of General Chemistry identify electronegativity as, for example, the ability of an atom to attract toward itself the electrons in a chemical bond, or an atom’s ability to compete for electrons with other atoms to which it is bonded. The same problem arises: How we can measure those abilities? This work aims to warn that details concerning electronegativity definition in different scales, usually not discussed in text books, are of remarkable importance.

Most of the textbooks indicate that different electronegativity scales have been proposed throughout the years and introduce two or three of them. Thus, textbooks inform that Linus Pauling was one of the first to provide a quantitative basis for electronegativity. Nevertheless, only few books indicate that Pauling’s electronegativity scale (PE) is not based on a direct measure of bond electron density (BED) displacement between two bonded atoms, but on thermal data interpreted on a simple quantum mechanical basis. Thus, PE difference between two atoms, A and B, is related (by a non-linear relationship) to the difference between a certain bond energy A-B and that predicted as a measure of those of A-A and B-B bonds. Lack of mathematical expressions in textbooks gives rise to misconceptions, even for this popular electronegativity scale. Thus, it is common that PE values are presented in textbooks as dimensionless, but are they really dimensionless or should they be expressed in eV1/2?

Mulliken’s ad Allred-Rochow’s are other two very popular scales (ME and ARE, respectively). ARE relates electronegativity to the attraction force between a certain nucleus and an electron in the covalent bond. This force is estimated by means of the effective nuclear charge and the covalent radius. Thus we cannot speak of experimental values for ARE electronegativity.

ME is the only scale usually accompanied by a quantitative expression in textbooks: the arithmetic mean of ionization potential and electron affinity. Unfortunately, this is not the original definition provided by Mulliken but a finite-difference approximation arising whithin the framework of the conceptual Density Functional Theory (DFT) from Iczkowski and Margrave definition.

We highlight that PE, ARE, and ME are all of them based on energetic considerations and none of them is directly related to the amount of BED belonging to each bonded atom. This consideration also opens a subsequent important question: what is an atom within a molecule? We believe Chemistry students should learn soon that chemical concepts are usually still far from being unambiguously defined. Moreover, introducing and discussing differences between diverse definitions may be very beneficial for them.