Question

Describe structure of the following molecules. 

i. Boron trichloride 

ii. Aluminium chloride 

iii. Orthoboric acid

Answer 1

i. Structure of boron trichloride (BCl₃) molecule:

• Boron trichloride (BCl₃) is a covalent compound. 
• In BCl₃ molecule, boron atom is sp hybridized having one vacant unhybridized p orbital. 
• B in BCl₃ has incomplete octet. 
• BCl₃ is a nonpolar trigonal planar molecule. 
• Each Cl – B – Cl bond angle is 120°

ii. Structure of aluminium chloride (AlCl₃) molecule:

• Aluminium atom in aluminium chloride is sp² hybridized, with one vacant unhybrid p-orbital. 
• Aluminium chloride exists as the dimer (Al₂C₆ ) formed by overlap of vacant 3d orbital of Al with a lone pair of electrons of Cl.

iii. Structure of orthoboric or boric acid (H₃BO₃) molecule:

• Orthoboric acid has central boron atom bound to three -OH groups. 
• The solid orthoboric acid has layered crystal structure in which trigonal planar B(OH)₃ units are joined together by hydrogen bonds.