Beryllium, the first member of the Group 2 metals, shows anomalous behaviour as compared to magnesium and rest of the members of the group. Further, it shows a diagonal relationship to aluminium.
Before going into the topic we must know what is diagonal relationship.? The answer to this question is quite simple. The similarity in properties existing between the diagonal pairs of elements such as Li-Mg, Be-Al is called a diagonal relationship.
Anomalous Behaviour between Beryllium and Other Elements
(a) Beryllium has an exceptionally small atomic and ionic size and thus does not compare well with other members of the group.
Because of the high ionization enthalpy and small size it forms compound, which is largely covalent and gets easily hydrolyzed.
(b) Beryllium does not exhibit a 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.
(c) The oxide and hydroxide of beryllium, unlike the hydroxides of other elements in the group, are amphoteric in nature.
(d) Beryllium hydride is electron deficient and polymeric, with multi-center bonding like aluminium hydride.
(e) The most unusual oxygen containing complexes of Be have the formula Be4O(O2CR)6 and are formed by refluxing Be(OH)2 with carboxylic acids.
These white crystalline compounds are soluble in non-polar organic solvents, such as alkanes. But they are insoluble in water and lower alcohols. In a solution, the compounds are unionized and monomeric.
The central oxygen atom is tetrahedral surrounded by the four Be atoms and each Be atom is tetrahedrally surrounded by four oxygen atoms. The six acetate groups are arranged along the six edges of the tetrahedral ion.
To summarise it in short, we can say that the anomalous behaviour of beryllium is due to
- High polarizing power.
- Its exceptionally small size.
- High Ionization energy.
- High electronegativity.
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:
(a) Like aluminium, beryllium is not readily attacked by acids.
Why beryllium is not attacked by an acid easily?
(b) Beryllium hydroxide dissolves in excess of alkali to give a beryllate ion, [Be(OH)4]2– just as aluminium hydroxide gives the aluminate ion, [Al(OH)4]–
(c) The chlorides of both beryllium and aluminium have Cl– bridged chloride structure in the vapour phase.
(e) Beryllium and aluminium ions have a strong tendency to form complexes, BeF42– and AIF63– respectively.