Question

How can you explain the anomalous behaviour of beryllium.

Answer 1

The very small size of the Li+ and Be2+ ions means that they have a large charge to size ratio and so show anomalous behaviour. The Li+ ion has a charge to size ratio comparable to that of Mg2+, and hence the properties of some lithium compounds resemble those of magnesium compounds.Many of these anomalous properties arise because  the salts of Li+ with small anions are exceptionally stable because of their very high lattice energies, whereas the salts with large anions are relatively unstable due to the low lattice enthalpies brought about by the poor packing of very large and very small ions. As a result of this, LiH is stable to 900oC, whereas NaH decomposes at 350oC; Li3N is stable, but Na3N does not exist at 25oC; LiOH decomposes to Li2O at redheat, but the other MOH sublime unchanged.The small size of the Li+ ion means that it has a very high enthalpy of hydration, and so lithium salts are much more soluble than the salts of other group 1 metals, eg. LiClO4 is up to 12 times more soluble than NaClO4.Beryllium has a unique chemical behaviour due to its high charge to size ratio, which is greater than that for any ion except H+ and B3+, with predominantly covalent chemistry, eg. its hydroxide can be precipitated from aqueous solution. The closest charge to size ratio to that of Be2+ is that of Al3+, and there are similarities in the chemistry.Due to the small size, high ionization energy, and high sublimation energy of beryllium, its lattice and hydration energies are insufficient to provide complete charge separation and the formation of simple Be2+ ions. In all compounds therefore, even BeO and BeF2, there is substantial covalent character in the bonding.Simple binary compounds tend to have four coordinate beryllium, eg. the structure of BeF2 is that of vertex-shared BeF4 tetrahedra, with bicoordinate fluoride ions, while BeCl2 forms chains of edge-shared BeCl4 tetrahedra with three coordinate end-groups terminating the chains. This tendency for low coordination numbers is not repeated in the rest of group 2, which form octahedrally coordinated compounds.Compounds similar in structure to the beryllium halides are formed with alkoxy groups, eg. [Be(OCH3)2]n is a polymer insoluble in hydrocarbon solvents, but again these are such as to give the beryllium atom a coordination number of four.