Account for the following observations
(a) `AlCl_(3)` is a Lewis acid
(b) Though fluorine is more electronegative than chlorine yet `BF_(3)` is a weaker Lewis acid than `CI_(3)`
(c) `PbO_(2)` is a stronger oxidising agent than `SnO_(2)`
(d) The +1 oxidation state of thallium is more stable than its +3 state.

Explanation

(i) The `Al` atoms in `AlCl_(3)`, has only six electron in tis valence shell and hence can accept apair of its octet. Therefore, it acts as a Lewis acid.
(ii) The B atom in `BF_(3)` or has only six electrons in the valene shell and hence can accept a pair of electrons to complete its octet. Therefore, both `BF_(3)` and `BCl_(3)` act as Lewis acids. But in `BF_(3)`, the size of empty 2p-orbital on B and 2p-orbital on F containing the lone pair of electrons of F is donated to the empty 2p-orbital to a considerable extent and hence electron deficiency of B decreases. In contrast, in `BCl_(3)`, the size of `3p`-orbital of Cl containing the lone pair .

of electron is much bigger than the empty 2p-orbital of B and hence donation of lone pair of electron of Cl to B does not occur to any significant extent. Therefore electron deficiency of B is much higher in `BCl_(3)` than that is `BF_(3)` and hence `BCl_(3)` is a stronger Lewis acid than `BF_(3)`.
(iii) N/A, (iv) N/A