All in all, our calculations on chalcogens show that a bonding change occurs in these elements as pressure increases. The A8 phase at RP features a mixture of primary covalent ppσ-bonds plus secondary bonds (related to LEPs), whereas the octahedrally-coordinated HP phases, such as the Ai structure and its slightly distorted structures, feature MMBs that are similar to those already discussed in PCMs, such as V2-VI3 chalcogenides, which are close to be isoelectronic to chalcogens. Polonium, in its two polymorphs at RP, is the only chalcogen with all bonds being fully MMBs at RP. Once established that MMBs occur in the octahedrally-coordinated crystalline phases pnictogens and chalcogens either by increasing pressure or by changing composition due to substitution by heavier elements of the same group, we will show next that the change from the pre-MMB scenario to the MMB scenario can be traced by the change of the number of electrons shared (ES) and the normalized number of electrons transferred (ET) in the two-center bonds of materials, as obtained from QTAIM methodology and used by Wuttig and coworkers . As already mentioned, the ES and ET parameters of a two-center bond provide an estimation of the degree of bond covalency and ionicity, respectively. In contrast to previous studies, we have calculated the pressure dependence of the ES and ET values in the A7 (A8) phases of group-15 (-16) elements not only for the primary covalent bond (with d1 length and plotted with solid lines in Fig. ), but also for the secondary bonds of those phases (with d2 length and plotted with dashed lines in Fig. ). Similarly, we have calculated the ES and ET values of the only bond present in the Ah and Ai phases in these two elemental families. In this way, we can see the evolution of the different bonds as MMB formation proceeds at HP. Since this work is mainly devoted to pure elements with a unique Wyckoff site at each crystalline structure, the ET values of the primary and secondary bonds are zero; thus, we only need to calculate the ES values of bonds in pnictogens and chalcogens at different pressures. As examples of both families, we show in Fig. the pressure dependence of ES in As and Se, as well as in α-Po and β-Po. The ES values of the primary covalent bonds in As-I and Se-I at RP are between 1.5 and 2, which are typical of the strong primary covalent bonds, while the ES values of the secondary bonds at RP (around 0.5) are typical of relatively weak secondary donoracceptor bonds . Remarkably, the ES value of the only bond present in the Ah and Ai phases of As and Se, respectively, is close to 1.0 (once the octahedral coordination is attained in As (Se) above 25 (23) GPa), which is already the value of ES for the two phases of Po at RP. These results mean that the ES values confirm the presence of MMBs in the octahedrally-coordinated phases of group-15 and -16 elements since it is proved that bonds in these phases are characterized by sharing one electron between two atoms, as already suggested by Wuttig and coworkers . In other words, MMB in PCMs as well as in the octahedrally-coordinated phases of pnictogens and chalcogens, is a kind of 2c-1e bond. However, it must be stressed that they are not isolated 2c-1e bonds but interacting 2c-1e bonds, as shown by the multicenter character of this unconventional bond . The use of the ES and ET values for primary bonds allows locating the different phases of pnictogens and chalcogens in the 2D map proposed by Wuttig and coworkers to show metavalency (see Fig. ) . As expected, the ES values of the primary covalent bonds in As, Sb, Se, and Te at RP are between 1.4 and 2.0. Meanwhile, the ES value of the primary bond in Bi at RP is 1.2; a value close to 1.0 that corresponds to materials showing MMBs, such as β-GeTe, SnTe, and PbS (PCMs with rs structure) . This result shows that Bi is characterized by bonds that are intermediate between covalent bonds and MMBs. This result is consistent with the appearance of asymmetric MMBs in Bi-II above 2.5 GPa (the smallest pressure of all pnictogens), as already commented. Finally, the ES values of the sixfold-degenerated bonds in α-Po and β-Po at RP, as well as in the HP phases of the other elements (As-II, sc-Sb, Bi-II, and β-Po phases of Se and Te) are all around 1.0 (see Fig. and Fig. ), thus evidencing that all these phases show MMBs that are in good agreement with their positions in the 2D map . In this context, arrows in Fig. indicate the direction followed by materials with a mixture of primary covalent bonds and secondary bonds when pressure is applied, or composition is changed for heavier elements within a group.