On mixing two colourless gases,a deep brown colour is observed. The gases are

Given below are two statements:
Statement-$I$: Element '$X$' and '$Y$' are the most and least electronegative elements,respectively,among $N, As, Sb$ and $P$. The nature of the oxides $X_{2}O_{3}$ and $Y_{2}O_{3}$ is acidic and amphoteric,respectively.
Statement-$II$: $BCl_{3}$ is covalent in nature and gets hydrolysed in water. It produces $[B(OH)_{4}]^{-}$ and $HCl$ in aqueous medium.

Which of the following elements shows a maximum valency of $5$?

Chemical nature of the nitrogen oxide compound obtained from a reaction of concentrated nitric acid and $P_{4}O_{10}$ (in $4:1$ ratio) is:

What is the chemical formula of hypophosphorus acid?

Nitrogen forms only $NCl_3$,whereas phosphorus forms both $PCl_3$ and $PCl_5$. Why?