Account for the following observations.

1.  AlCl3 is a Lewis acid.

2.  Though fluorine is more electronegative than chlorine yet BF3 is a weaker Lewis acid than CI3.

3.  PbO2 is a stronger oxidising agent than SnO2.

4.  The +1 oxidation state of thallium is more stable than its +3 state.

(a) In AlCl3, Al has only six electrons in its valence shell. It is an electron deficient species. Therefore, it acts as a Lewis acid (electron acceptor)
(b) In BF3 boron has a vacant 2p-orbital and fluorine has one 2p-completely filled unutilised orbital. Both of these orbitals belong to same energy level therefore, they can overlap effectively and form $\mathrm{p\pi }-\mathrm{p\pi }$ bond. This type of bond formation is known as back bonding.
While back bonding is not possible in BCl3 because there is no effective overlapping between the 2p-orbital of boron and 3p-orbital of chlorine. Therefore, electron deficiency of B is higher in BCl3 than that of BF3. That is why BF3 is a weaker Lewis acid than BCl3.
(c) In PbO2 and SnO2, both lead and tin are present in +4 oxidation state. But due to stronger inert pair effect, Pb2+ ions is more stable than Sn2+ ion. In other words, Pb4+ ions i.e., PbO4 is more easily reduced to Pb2+ ions than Sn4+ ions reduced Sn2+ ions. Thus, PbO2 acts as a stronger oxidising agent than SnO2
(d) Tl+ is more stable than Tl3+ because of inert pair effect.