${\mathrm{BCl}}_3$ is trigonal planar while ${\mathrm{AlCl}}_3$ is tetrahedral in dimeric state. Explain.

Explain the following:

Boron has high melting and boiling point.

Explain the following:

Boron does not form ${\mathrm{B}}^{3+}$ ion.

Explain the following:

${\mathrm{AlCl}}_3$ forms a dimer but ${\mathrm{BCl}}_3$ does not form dimer.

Explain the following:

Boron and aluminium halides behave as Lewis acids.