In 1916, Lewis and Kossel laid the empirical ground for the electronic theory of valence, whose quantum theoretical foundation was uncovered only slowly. We can now base the classification of the various traditional chemical bond types in a threefold manner on the one-and two-electron terms of the quantum-physical Hamiltonian (kinetic, atomic core attraction, electron repulsion). Bond formation is explained by splitting up the real process into two physical steps: (i) interaction of undeformed atoms and (ii) relaxation of this nonstationary system. We aim at a flexible bond energy partitioning scheme that can avoid cancellation of large terms of opposite sign. The driving force of covalent bonding is a lowering of the quantum kinetic energy density by sharing. The driving force of heteropolar bonding is a lowering of potential energy density by charge rearrangement in the valence shell. Although both mechanisms are quantum mechanical in nature, we can easily visualize them, since they are of one-electron type. They are however tempered by two-electron correlations. The richness of chemistry, owing to the diversity of atomic cores and valence shells, becomes intuitively understandable with the help of effective core pseudopotentials for the valence shells. Common conceptual difficulties in understanding chemical bonds arise from quantum kinematic aspects as well as from paradoxical though classical relaxation phenomena. On this conceptual basis, a dozen different bond types in diatomic molecules will be analyzed in the following article. We can therefore examine common features as well as specific differences of various bonding mechanisms. Key words: bond types; covalence; polar bond; bond energy analysis; diatomics; effective core potentials; pseudopotentials Assessment of Objective ModelsChemistry, the art of changing materials on the macroscopic scale, requires various kinds of tools for explaining and predicting these processes on the atomistic and electronic scale. We can distinguish tools for three different aims: (i) tools for the quantitative numerical determination and accurate prediction of energies, forces, velocities, etc. for individual cases (by experimental or computational or mixed procedures); (ii) tools for the detailed description of empirical findings with the aim of a practically useful classification of phenomena, and for more general and qualitative predictions; and (iii) tools for an intuitive understanding of the formation of different kinds of compounds with different kinds of bonds and properties. ''It is important to understand the reasons behind the facts in a physical way. '' 1 This not only satisfies a fundamental philosophical desire but also furnishes valuable, if loose, guidance for practical work.Understanding chemical bonds means to be able to qualitatively anticipate the mechanism and outcome of physical processes that lead from separated atoms or molecular fragments to energetically stabilized (quasi-)stationary compounds. This can be achieved best with the help o...
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