10.8 Anomalous Behaviour Of Beryllium

Beryllium, the first member of the Group 2 metals, shows anomalous behaviour as compared to magnesium and rest of the members. Further, it shows diagonal relationship to aluminium which is discussed subsequently.

Beryllium has exceptionally small atomic and ionic sizes and thus does not compare well with other members of the group. Because of high ionisation enthalpy and small size it forms compounds which are largely covalent and get easily hydrolysed.

Beryllium does not exhibit coordination number more than four as in its valence shell there are only four orbitals. The remaining members of the group can have a coordination number of six by making use of d-orbitals.

The oxide and hydroxide of beryllium, unlike the hydroxides of other elements in the group, are amphoteric in nature.

 

10.8.1 Diagonal Relationship between Beryllium and Aluminium

The ionic radius of Be2+ is estimated to be 31 pm; the charge/radius ratio is nearly the same as that of the Al3+ ion. Hence beryllium resembles aluminium in some ways. Some of the similarities are:

Like aluminium, beryllium is not readily attacked by acids because of the presence of an oxide film on the surface of the metal.

Beryllium hydroxide dissolves in excess of alkali to give a beryllate ion, [Be(OH)4]2– just as aluminium hydroxide gives aluminate ion, [Al(OH)4]–.

The chlorides of both beryllium and aluminium have Cl– bridged chloride structure in vapour phase. Both the chlorides are soluble in organic solvents and are strong Lewis acids. They are used as Friedel Craft catalysts.

Beryllium and aluminium ions have strong tendency to form complexes, BeF4 2–, AlF6 3–.

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