Whether Electronegativity and Hardness Are Manifest Two Different Descriptors of the One and the Same Fundamental Property of Atoms—A Quest DULAL C. GHOSH, NAZMUL ISLAM Department of Chemistry, University of Kalyani, Kalyani 741235, India Received 8 June 2009; accepted 6 July 2009 Published online 00 Month 2009 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/qua.22415 ABSTRACT: In this report, we have tried to reveal that there is much conceptual commonality between the two fundamental theoretical descriptors of chemistry and physics—the electronegativity and the hardness. The physical hardness was introduced and theorized by condensed matter physicists. The chemical hardness was introduced by chemists to generalize and rationalize the HSAB principle. We have tried to establish that the physical hardness and the chemical hardness with evolution of time have converged to one and the same general principle—the hardness. We have also tried to understand the physical basis and operational significance of another very important descriptor arising out of theoretical constructs of chemistry—the electronegativity. We have relied upon the fact that, since these descriptors are not observables, there is no possibility of their quantum mechanical evaluation. These descriptors, therefore, should be and must be reified before suggesting ansatz for their evaluation. We have dwelt at length upon the effort of density functional definition and evaluation of electronegativity and hardness and discovered the inherent inner contradiction of the theory and measurement. We have also noted that a good number of scientists hold the opinion that the density functional formula of electronegativity is  = I and that of hardness is  = I, where I is the ionization potential of the chemical system. This study concludes that the two fundamental descriptors—the hardness and the electronegativity originate from the same source—the electron attracting power of the screened nucleus upon valence electrons and discovers the surprising result that if one measures hardness, the electronegativity is simultaneously measured and vice versa. We have also explored the ansatz of semiempirical evaluation of electronegativity and hardness Correspondence to: D. C. Ghosh; e-mail: dcghosh1@rediffmail. com International Journal of Quantum Chemistry, Vol 000, 000 – 000 (2009) © 2009 Wiley Periodicals, Inc.