Moreover, the strengthening of the Rb-Rb bonds in the B2 phase goes to the detriment of the Rb-F interaction, which is weaker than in the B1 phase. These trends are explainable by the symmetry and the orientation of the p-orbitals in B2, as shown in Figure , where the lobes of the metal p-orbitals are aligned with the Rb-Rb contacts. This aspect becomes even more evident from the topological analysis of the electron density (Table and Figure ). Only in the B2 phase allows for a free-paths along M-M contacts, while in the B1 phase, the path must pass through the fluorine atoms lying in the same plane. Topologically, this is translated by the presence of bond-critical points (bcps) and charge concentrations along the M-M contacts in the B2 phases (Table ), and by ring-critical points (rcps) in the B1 phases. The topological descriptors at the M-M bcps, characterize this interaction similarly to van der Waals or hydrogen bonds. This topological description is analogous to the results provided in the extensive study done by Pendás et al., where they also suggested that, according to the topological analysis of the electron density, several B2 alkali halides appear as 14 coordinated (8 M-F contacts plus 6 M-M contacts). Therefore, orbital-based and electron density-based chemical bonding descriptors suggest that K, Rb and Cs, in the B2 alkali fluoride phases, behave like 7 valence electrons elements. In this model, the outer valence electrons (4s 1 , 5s 1 and 6s 1 ) are transferred from the three heavy alkali metals to the fluorine atoms. While the semi-core electrons (3p 6 , 4p 6 and 5p ), polarize and form intermetallic bonding interactions without the necessity of their oxidation.