Anomalous Behaviour of Beryllium
Beryllium , the first member of alkaline earth metals, show an anomalous behaviour , i.e. differ from the rest of the members of its family. The main reason for this difference are as follow:
1) Exceptionally small atomic and ionic size.
2) High ionisation enthalpy
3) Absence of d-orbital in its valence shell
Some important properties in which beryllium differ from the rest of the members of its group are as follow:
1) Beryllium is harder than other members of its group.
2) Beryllium has higher melting and boiling point than other members of its group.
3) Beryllium does not react with water even at high temperature while other metals do.
Mg + H2O ——> MgO + H2
4) Beryllium forms covalent compounds whereas other members form ionic compounds.Because of covalent character , salts of beryllium are easily hydrolysed.
5) Beryllium oxide and hydroxide are amphoteric whereas oxides and hydroxides of other alkaline earth metals are basic.
BeO + 2 HCl ——–> BeCl2 + H2O
BeO + 2 NaOH ——> Na2BeO2 + H2O
6) Carbides of Be are covalent whereas carbides of other members are ionic.
7) Beryllium carbide reacts with water to produce methane gas whereas carbide of other alkaline earth metals give acetylene gas.
BeC + 4 H2O ——–> 2 Be(OH)2 + CH4
CaC2 + 2 H2O ——> Ca (OH)2 + HC≡CH
8) Beryllium does not exhibit coordination number of more than four since its valence shell has only 4 orbitals; one s and p.The remaining elements of this group can show a coordination number of six by making use of d- orbitals in addition to s and p orbitals.