(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
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