Question

why fcl3 does not exist

Answer 1

There are particularly two reasons:

Cl is a big atom and F is small. So while Cl can hold 3 F atoms but F cannot do the same.Cl has d orbitals while F does not. Note that all halogens have p-5 configuration so for higher valency each atom should unpair its paired electrons and promote it to d orbital. In absence of so, F can't show any valency of more than 1 causing non formation of FCl3.

Answer 2

There are particularly two reasons:

Cl is a big atom and F is small. So while Cl can hold 3 F atoms but F cannot do the same.Cl has d orbitals while F does not. Note that all halogens have p-5 configuration so for higher valency each atom should unpair its paired electrons and promote it to d orbital. In absence of so, F can't show any valency of more than 1 causing non formation of FCl3.