Aqueous solution of a salt $$(A)$$ forms a dense white precipitate with $$\mathrm{BaCl}_2$$ solution. The precipitate dissolves in dilute $$\mathrm{HCl}$$ to produce a gas $$(B)$$ which decolourises acidified $$\mathrm{KMnO}_4$$ solution. $$A$$ and $$B$$ respectively are

Phosphorus pentachloride

The last element of the $$p$$-block in 6 th period is represented by the outer most electronic configuration.

A Lewis acid '$$X$$' reacts with $$\mathrm{LiAlH}_4$$ in ether medium to give a highly toxic gas. This gas when heated with $$\mathrm{NH}_3$$ gives a compound commonly known as inorganic benzene. The gas is