The electronegativity and the hardness are two different fundamental descriptors of atoms and molecules, and this chapter describes how the authors have logistically discovered the commonality between the heuristic and basic philosophical structures of their origin and also the manifestation in the real world. Also, the chapter demonstrates that the physical hardness and the chemical hardness with evolution of time have converged to one and the same general principle– the hardness. The authors also try to expose the physical basis and operational significance of another very important descriptor–the electronegativity. The chapter also explores whether the hardness equalization principle can be conceived analogous to the well established electronegativity equalization principle. The authors hypothesize that the electronegativity and the absolute hardness are two different appearances of the one and the same fundamental property of atoms, and the Hardness Equalization Principle can be equally conceived like the electronegativity equalization principle. To test this hypothesis, the authors have made several comparative studies by evaluating some well known chemico-physical descriptors of the real world, such as hetero nuclear bond distances, dipole charges, and dipole moments of molecules. The detailed comparative study suggests that the paradigm of the hardness equalization principle may be another law of nature like the established electronegativity equalization principle.
Charge conservation in electronegativity equalization and its implications for the electrostatic properties of fluctuating-charge models
