The Full Story of the Electron Configurations of the Transition Elements

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)dG−1ns1 or (n − 1)dG−2ns2. Five features must be considered to resolve this paradox. Point (i), the d−s orbital−energy 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 spin−orbit 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.