(i) Aluminium has 3 electrons in its valence shell ([Ne]3s²3p¹) which are incomplete and chlorine has electronic configuration of ([Ne3s²3p⁵]),In trivalent state, the central atom in a molecule of a compound has only 6 electrons and such deficiency of electron have a tendency to accept the pair of electron to achieve stable electronic configuration and behave as a Lewis acid.

(ii) Boron, ([He]2s²2p¹) in BF₃has incomplete 2p orbital whereas fluorine ([He]2s²2p⁵) has completely filled 2p orbital. BF₃ being electron deficient are strong Lewis acid and also BF₃ reacts easily with Lewis base like NH₃ to complete the octet around boron but the aptness to behave as Lewis acid decrease with increase in size down the group.BCl₃ accept lone pair of ammonia to form BCl₃.NH₃.

(iii) Sn(tin) decompose steam to form SnO₂ and hydrogen

Sn + 2H₂O → SnO₂ + 2H₂

But lead forms protective oxide film when reacting with water.

That is why PbO₂ is more oxidising agent than SnO₂.

(iv)Thallium, with electronic configuration of [Xe]4f¹⁴5d¹⁰6s²6p¹ and has both +1 and +3 oxidation state. +1 oxidation state is more dominant but +3 oxidation sate is highly oxidising in nature. The stability of +1 oxidation state increases for heavy element, thus +1 oxidation state is more stable than +3 oxidation state.

Al <Ga< In <Tl