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

$A$ metal $(X)$ dissolves in both dilute $HCl$ and dilute $NaOH$ to liberate $H_2$ gas. Addition of $NH_4Cl$ and excess $NH_4OH$ to an $HCl$ solution of $(X)$ produces $(Y)$ as a precipitate. $(Y)$ is also produced by adding $NH_4Cl$ to the $NaOH$ solution of $(X)$. The species $(X)$ and $(Y)$,respectively,are:

Identify the correct statement.

Which of the following acids forms two series of salts?

The most stable trihalide of nitrogen is ..... .

The element with atomic number $33$ belongs to which group of the periodic table?