1. The A7 structure of group-15 (As, Sb, Bi) elements tends at HP towards the Ah phase of α-Po at RP, while the A8 phase of group-16 (Se, Te) elements tends at HP towards the Ai phase of β-Po at RP. These results are consistent with the observation of α-Po and β-Po as HP phases in group-15 (P, As) and group-16 (S, Se, Te) elements, respectively, and agree with the electron count of Papoian and Hoffmann on the formation of hypercoordinated units . The formation of the cubic α-Po structure in group-15 elements is favored by their five valence electrons, while the formation of the rhombohedral β-Po structure in group-16 elements with six valence electrons occurs because the extra valence electron avoids the complete inactivation of the LEP at HP and induces a distortion in the cubic structure. The reason why α-Po occurs at RP and low temperatures could be related to the small ligand-ligand repulsion between Po atoms at low temperatures compared to the energy gained when the LEP is promoted to the valence shell.