Based on the Born-Haber cycle, a method is proposed for determining the thermochemical radii and enthalpies of the formation of polyatomic ions. In order to be able to compare the values of the enthalpies of the formation of ions and salts, a method is proposed for normalizing these values by relating them not to one mole of a substance, but to an averaged gram atom. The method is approved on vanadium oxide compounds. The values of 22 radii and enthalpies of the formation of polyatomic ions containing vanadium, the energies of the crystal lattices and the enthalpies of formation of 81 vanadate are calculated. The obtained values are compared with the experimental data
