Boron trihalides are Lewis acids. The order of their acidic strength is BF3<BCl3<BBr3<BI3 In boron halides, a pπ−pπ back bonding arises due to empty orbital of boron and filled orbitals of halogen. This pπ−pπ bonding has maximum effect in BF3 as the size of B and F atoms are comparative. This effect decreases as the size of halogen atom increases. Due to this effect, tendency of accepting lone pair of electron of boron decreases, i.e. Lewis acid character decreases.