Electronegativity and Chemical Hardness: Different Patterns in Quantum Chemistry

Five levels for emphasizing the electronegativity and chemical hardness quantum chemical differences were reviewed: the analysis of their quantum definitions in Fock space, their reliability in providing atomic patterns of periodicity within density functional softness as well as the semiclassical quantum propagator/Green function based theories as compared to the traditional finite difference methods based on ionization potential and electronic affinity, in providing viable relationships with other aromaticity energetic, magnetic, topologic and geometric criteria, and exposing their sensitivity to the media or solvent effects. The revealed differences reflect the special role electronegativity and chemical hardness play in modeling chemical reactivity and chemical bonding by associating fundamental principles along with the conceptual and computational quantum methods in approximating, implementing, and combining the energy and density quantities and related properties of a multi-electronic multi-nuclei system; still, based on revealed quantum differences, the associate hierarchy of principles in chemical bonding is established and the idea of chemical orthogonal space having electronegativity and chemical hardness as main vectors is advanced.