Towards understanding the driving forces of the formation of multicomponent compounds: The case of complex oxides

The strongest driving forces for the formation of binary compounds (e.g., Na2O, MgO, Al2O3, and SO3) are related to the octet rule (formation of closed electronic shells) and charge redistribution as a result of electronegativity differences. Here, we investigate the driving forces behind the reactions of these binary compounds with each other, traditionally described in the language of acid-base interactions. For example, why do Na2O and SO3 (both of which have a closed-shell electronic structure) react with each other forming Na2SO4? In addition to Na2SO4, we also consider the processes of formation of Mg2SiO4 and MgAl2O4, tracking changes in chemical bonding characteristics and ionic charges of the constituent atoms. We show that in such acid-base reactions, electrons move from the atomic states with higher energies (from the basic oxide-forming element) to atoms with lower-energy states. This happens by changing the degrees of ionicity of bonds, without disrupting the closed-shell electronic structure.