Chlorine has vacant 3d-orbitals and an electron can be promoted from 3p filled orbital to 3d orbital. Thus, chlorine can show a cavalency of 3 in its compunds and a molecule of `ClF_(3)` can exist. But fluroine has no vacant 2d orbitals and can exhibit only cavalency of one in its compounds. Thus, a molecule of `FCl_(3)` fails to exist.
