The tendency of BF ₃, BCl ₃ and BBr ₃ to behave as Lewis acid decreases in the sequence

Step-by-step solution

As the size of halogen atom increases, the acidic strength of boron halides increases. Thus, BF ₃ is the weakest Lewis acid. This is because of the p - p back bonding between the fully-filled unutilised 2 p orbitals of F and vacant 2 p orbitals of boron which makes BF ₃ less electron deficient. Such back donation is not possible in case of BCl ₃ or BBr ₃ due to larger energy difference between their orbitals. Thus, these are more electron deficient. Since on moving down the group the energy difference increases, th