Phillips J C. Ionicity of the chemical bond in crystals. Rev. Mod. Phys. 42:317-56, 1970

“In 1963 Marvin Cohen and I, at the University of Chicago, had been studying the energy bands of semiconductor crystals and using their fundamental optical absorption spectra to refine the results of quantum mechanical calculations when we realized that our deductive approach left unanswered some of the most important questions, for both the calculations and the experiment showed systematic chemical trends in bonding which were not easily identified in the starting potentials of the separated atoms. Of course, I knew that some of these trends could be described roughly by Linus Pauling’s atomic electronegativity parameters, which he defined as the ability of an atom in a bonded state to attract electrons. But other trends depended on atomic size in a way that I could not determine, and many trends depended on both size and electronegativity in some unknown way. “At this time I became quite discouraged about my ability to Phillips J C. Ionicity of the chemical bond in crystals. Rev. Mod. Phys. 42:317-56, 1970. [Bell Telephone Laboratories, Murray Hill, NJ] CC/NUMBER 1 JANUARY 5, 1981