CBSE Class 12-science Answered

Ground state configuration of B: 1s² 2s² 2p¹.

Since the formation of three bonds with chlorine atoms require three unpaired electrons, there is the promotion of one of the 2s electron into the 2p sublevel by absorbing energy.

So the configuration becomes 1s2 2s1 2px12py1.

Boron undergoes sp² hybridization by using a 2s and two 2p orbitals to give three half-filled sp²hybrid orbitals which are oriented in trigonal planar symmetry.

Boron forms three σsp-p bonds with three chlorine atoms by using its half-filled sp² hybrid orbitals. Each chlorine atom uses it's half filled p-orbital for the σ-bond formation.  Thus the shape of BCl₃ is trigonal planar with bond angles equal to 120^o.