Question

Why 3p orbital of chlorin is used in bcl3 instead of 2p?

Answer 1

The configuration of chlorine atom is:

1s² 2s² 2p6 3s² 3p5

So, as you can see there's one unpaired electron in the 3p orbital while the 2p orbital is completely filled.

So during hybridization only the 3p orbital can take part, not 2p.