Formation and structure of Boron Trichloride (BCI3) molecule: The electronic configuration of boron (5) is 1s2 2s2 2px1 2py0 2pz0. When one electron is transferred from 2s orbital to 2p orbital, it has three unpaired electrons, which explains the valency of three for boron. The sp2 hybridization takes place on boron atom.
At normal state, B (5): 1s2 2s2 2px1 2py0 2pz0
At excited state. B* (5): 1s2 2s1 2px1 2py0 2pz0
After sp2 hybridization, B (5): 1s2 ψ11 ψ21 ψ31
Where ψ1, ψ2 and ψ3 are hybrid orbitals. They overlap with 3pz orbitals (Containing one electron each) of three chlorine atoms as follows and form three B-CI bonds. Then BCl3 molecule is formed.
So, Boron Trichloride (BCl3) is a planar molecule, where all bond angles are 1200. Although each B-Cl bonds is polar, the vector summation of three dipoles is zero and Bell has a zero dipole moment.