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Diagonal Relationship of Boron with Silicon

Boron shows anomalous behaviour in its groups because of its small size and non-availability of d orbitals. It resembles silicon, the second member of the next higher group.
  1. Both boron and silicon are non-metals, and exist in allotropic forms. They have high melting points and are semiconductors. The other members of Group 13 are metals.
  2. Both boron and silicon form several volatile and spontaneously inflammable hydrides called boranes and silanes, respectively. The hydrides are readily hydrolyzed. The lower hydrides can be obtained by the reduction of chlorides with LiAlH4.
    4BCl3 + 3LiAlH4 3AlCl3 + 3LiCl + 2B2H6
    SiCl4 + LiAlH4 AlCl3 + LiCl + SiH4
    In contrast, aluminium hydride is a polymeric solid.
  3. Both boron and silicon form halides, which are readily hydrolyzed.
BCl3 + 3H2O B(OH)3 + 3HCl
                         Boric acid
SiCl4 + 4H2O  Si(OH)4 + 4HCl
                          Silicic acid
The aluminium halides are only partly hydrolyzed in water.
  1. B2O3 is an acidic oxide, like SiO2.
    B2O3 + 6NaOH 2Na3BO3 + 3H2O
    SiO2 + 2NaOH Na2SiO3 + H2O. 
    Al2O3 is an amphoteric oxide.
  2. Both boron and silicon react with several metals to form metal borides and silicides.
    This dimeric structure is retained in non-polar solvents such as benzene. In water, the high enthalpy of hydration is sufficient to break the covalent dimer into [M . 6H2O]3+ and 3X- ions.

3Mg + 2B Mg3B2
                     Magnesium boride

2Mg + Si Mg2Si               
                     Magnesium silicide
Borides and silicides are decomposed by dilute acids to give volatile hydrides (boranes, silanes).
  1. Both boron and silicon, as well as their oxides, react with alkalis to form borates and silicates containing BO4 and SiO4 tetrahedral units, respectively.

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