Hello Kumar Gaurav,

bcl3 is planner molecule because:

BCl3 has no lone pair of electrons. Boron has only 3 valence electrons, so when it bonds with cl there will only be 3 electron pairs around the boron atom. Repulsion theory predicts that these three e- pairs should find themselves at the vertices of an equilateral triangle (bond angles of 120 degrees). Thus, Bcl3 is planar triangular.

hope this is helpful.

All the best.

Hello!!

Bcl3 is a planar molecule whereas NCl3 is pyramidal because (a) nitrogen atom in smaller than boron atom (b) N– cl bond is more covalent than B– cl bond (c) B– cl bond is more polar than N–cl bond (d) BCl3 has no lone pair of electrons where NCl3 has a lone pair of electrons.

I took example of nitrogen for your better understanding

Thanks