The B atom in `BF_(3)`or `BBr_(3)` has only six electron inits valence shell and hence can accept a pair of electrons to complete its octet. Therefore, both `BF_(3)` and `BBr_(3)` at as Lewis acids. But in `BF_(3)`, the sizes of empty `2p-`orbital of B and the 2p-orbital of F containing the lone pair of electrons are almost identical and hence effective `ppi - ppi` bounding occurs. As a result, the lone pair of F is donate to B atom and hence the electrons deficiency of boron decreases . In contrast, in `BBr_(3)`. the size of 4p-orbital ov Br containing the lone pair of electrons is much bigger than the empty 2p-orbital of B and hence donation of lone pair of electrons of Br to B does not occur to an signification extent. As a result , the electron deficiency of B is much higher to `BBr_(3)` than in `BF_(3)`, and hence `BBr_(3)` i a stronger Lewis and than `BF_(3)`.
