Electronegativity from Avogadro to Pauling : II . Late Nineteenth

JChemEd.chem.wisc.edu • Vol. 80 No. 3 March 2003 • Journal of Chemical Education 279 Part I of this series traced the origins of the electronegativity concept in the writings of Amedeo Avogadro and Jöns Jacob Berzelius in the period 1809–1813 (1). Avogadro’s approach—based on the concepts of acidity and alkalinity as generalized relative properties, the use of contact or Volta potentials as a measure of relative electronegativity orders, and an awareness of the potential perturbations of mass–action effects in establishing relative affinity orders—was the more sophisticated of the two. However, the less rigorous approach of Berzelius, when coupled with his greater success in establishing a complete table of relative electronegativities for the then known elements and his far more influential reputation among European chemists of the period, soon eclipsed Avogadro’s more fundamental contribution. Unfortunately Berzelius proceeded to tie his version of the electronegativity concept to the oxygen-based, acid–base dualistic system of chemical classification originally introduced by Lavoisier, thereby converting it into a system of “electrochemical dualism”. Though this system worked well for simple inorganic compounds and had some initial success when applied to organic chemistry in the period 1820– 1839, it began to seriously unravel in the 1840s under the increasing strain of the bewildering array of new organic compounds and reactions being discovered. In a desperate attempt to bring order to the resulting chaos, chemists began to abandon the premises of electrochemical dualism one by one and to try a variety of new organizational schemes. The substitution theory, the unitary theory, the nucleus theory, the old type theory, and the new radical theory followed one another in rapid succession, with the new type theory emerging as the eventual victor in the 1850s (2). This, in turn, was generalized, via the newer concepts of valence and topological bonding in the 1860s, to form “structure theory”, which was finally completed in its classical form with the introduction of the tetrahedral carbon atom in 1874. As argued elsewhere, these latter events form the substance of the second chemical revolution of 1855–1875 (3). In the course of abandoning the electrochemical dualism of Berzelius, chemists tended to abandon the electronegativity concept as well. However, once structure theory began to take on its final form, they came to realize that the electro-negativity concept was in fact independent of the misleading compositional and structural implications of dualism and began to explore ways of reconciling it to the newer concepts of valence and chemical structure. In what follows we will trace this accommodation process as it occurred in four fundamental areas of chemistry in the period 1870–1910 (4):