Journal of Chemical Education, vol. 87, issue 4 (2010) pp. 444-448
The dominant electronic valence configurations of atoms in chemical substances of a transition element of group G in period n is (n 1)dGns0. Transition-metal chemistry is d orbital chemistry. In contrast, the ground states of free, unbound atoms derive, in most cases, from configurations (n 1)dG1ns1 or (n 1)dG2ns2. Five features must be considered to resolve this paradox. Point (i), the ds orbitalenergy distance in relation to the averaged differences of dd, ds, and ss two-electron repulsion energies, has often been discussed in the literature. However, four equally important features are (ii) the different two-electron repulsion energies within the d shell; (iii) the spinorbit coupling energies in the d shell;and, in particular, (iv) the d-orbital collapse below the s level for increasing nuclear charge around group 3 and (v) the different perturbations of the valence (n 1)d and ns orbitals when a free atom enters an alloy or compound. There is a conceptual difference between spectroscopic ground states and chemically dominant ground configurations. The ground states of unbound atoms mentioned in most chemistry textbooks have little meaning in chemistry.
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