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