The reasons for the difference in coordination numbers of the central atoms (E) in the oxyanions of main-group elements of the second and third periods have been considered using various quantum-chemical methods. Isoelectronic series—triangular EO 3 and tetrahedral EO 4 —have been studied both in the isolated state (ions) and with inclusion of the protonic and cationic environment (acids and salts). Simulation of the elementary act of addition reactions like H k EO 3 + H 2 O = H k + 2 EO 4 (even in this most simplified form) demonstrates that in the case of E(II), these (often strongly endothermic) reactions do not proceed, whereas in the case of E(III) they proceed without barrier. The available few deviations from this experimental fact have been examined. Possible more complex approaches with inclusion of the environment (primarily, water) have been discussed, and the role of various H-bonds has been analyzed. © 2019, Pleiades Publishing, Ltd.