A colourless, neutral, paramagnetic oxide of nitrogen '$$P$$' on oxidation gives reddish brown gas $$Q$$. $$Q$$ on cooling gives colourless gas $$R$$. $$R$$ on reaction with $$P$$ gives blue solid $$S$$. Identify $$P, Q, R, S$$ respectively

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.