Dimeric structure of aluminium chloride
– Boron halides do not form dimers because the size of boron is so small that it is unable to coordinate four large-sized halide ions .
Anomalous properties of boron
1 . Boron is a non-metal & bad conductor of electricity whereas aluminium is a metal & good conductor . B is hard but Al is a soft metal .
2 . Boron exists in two forms-crystalline and amorphous . But Al does not exist in different forms .
3 . The melting and boiling point of boron are much higher than that of Al .
4 . Boron forms only covalent compounds whereas Al forms even some ionic compounds .
5 . The hydroxides and oxides of boron are acidic in nature whereas those of aluminium are amphoteric .
6 . The trihalides of boron exist as monomers . On the other hand , aluminium halides exist as dimers .
7 . The hydrides of boron are quite stable while those of aluminium are unstable
• Boron and silicon exhibit the typical properties of non-metals . These do not form cations . Both exist in amorphous as well as crystalline forms .
• Boron oxide ( B2O3 ) and silica ( SiO2 ) both are acidic and dissolve in alkali solutions to form borates and silicates respectively .
B2O3 + 6NaOH → 2Na2BO3 + 3H2O SiO2 + 2NaOH → Na2SiO3 + H2O