Beryllium can be thought of as being even more covalent than magnesium. The small size (ca. 0.3 Å) results in a very high charge density of Be2+. In addition, the ionization energy for beryllium is a large positive value (1st ionization energy = 899.5 kJ/mol, 2nd ionization energy = 14,848.7 kJ/mol). Both of these factors means that the free ion does not exist. Instead, beryllium forms covalent compounds in a similar manner to its diagonal analog aluminum. Both beryllium and aluminum form covalent compounds or strongly solvated cations, and both form polymeric hydrides, chlorides, and alkyls.
Beryllium chloride is not a lattice structure with a concomitantly high melting and boiling point as observed for the other Group 2 metals (Table 1). Instead BeCl2 is a polymer in the solid state (Figure 1a), and an equilibrium between a monomer (Figure 1b) and dimer (Figure 1c) in the vapor phase.
| M | Structure |
| Be | Polymer (4-coordinate Be) |
| Mg | Cadmium chloride structure (6-coordinate Mg) |
| Ca | Deformed rutile structure (6-coordinate Ca) |
| Sr | Deformed rutile structure (6-coordinate Sr) |
| Ba | PbCl2 structure (9-coordinate Ba) or fluorite structure (8-coordinate Ba) |
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