>>
Study Material >>
IIT JEE Chemistry >>
Inorganic Chemistry >>
S and P-Block Elements >>
Anamolous Behaviour of Beryllium
Anamolous Behaviour of Beryllium
The properties of berrylium the first member of the alkaline earth metal, differ from the rest of the member. Its is mainly because of
(i) Its small size and high polarizing power.
(ii) Relatively high electro negativity and ionization energy as compared to other members.
(iii) Absence of vacant d – orbitals in its valence shell.
Some important points of difference between beryllium and other members (especially magnesium) are given below:
(i) Be is harder than other members of its group.
(ii) Be is lighter than Mg.
(iii) Its melting and boiling points are higher than those of Mg & other members.
(iv) Be does not react with water while Mg reacts with boiling water.
(v) BeO is amphoteric while MgO is weakly basic.
(vi) Be forms covalent compounds whereas other members form ionic compounds.
(vii) Beryllium carbide reacts with water to give methane whereas carbides of other alkaline earth metals gives acetylene gas.
Be2C + 4H2O → 2Be (OH)2 + CH4
Mg2C2 + 2H2O → Mg (OH)2 + C2H2
CaC2 + 2H2O → Ca (OH)2 + C2H2
(viii) Beryllium does not exhibit coordination number more than four as it has four orbitals in the valence shell. The other members of this group has coordination number 6.
Resemblance of Beryllium with Aluminium (Diagonal relationship)
The following points illustrate the anomalous behaviour of Be and its resemblance with Al.
(i) Unlike groups – 2 elements but like aluminium, beryllium forms covalent compounds.
(ii) The hydroxides of Be, [Be(OH)2] and aluminium [Al(OH)3] are amphoteric in nature, whereas those of other elements of group – 2 are basic in nature.
(iii) The oxides of both Be and Al i.e. BeO and Al2O3 are high melting insoluble solids.
(iv) BeCl2 and AlCl3 have bridged chloride polymeric structure.
(v) The salts of beryllium as well as aluminium are extensively hydrolysed.
(vi) Carbides of both the metal reacts with water liberating methane gas.
Be2C + 4H2O → 2Be (OH)2 + CH4
AI4C3 + 12H2O → 4Al (OH)3 + 3CH4
(vii) The oxides and hydroxides of both Be and Al are amphoteric and dissolve in sodium hydroxide as well as in hydrochloric acid.
BeO + 2HCI → BeCI2 + H2O
BeO + 2NaOH → Na2BeO2 + H2O
Al2O3 + 6HCI → 2AICI3 + H2O
AI2O3 + 2NaOH → 2NaAIO2 + H2O
(viii) Like Al, Be is not readily attacked by acids because of the presence of an oxide film.
Illustration:
Give the structure of BeCl2 in the
(i) Vapour state
(i) Linear molecule
(ii) Polymeric structure with bridged chlorine atom