White phosphorus contains

Nitrogen forms $N_2$,but phosphorus is converted from $P_2$ to $P_4$. Which of the following is the reason for this?

Nitric oxide $(NO)$ is prepared by the action of dilute $HNO_3$ on which of the following metals?

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

Which of the following is the structural formula of hypophosphorous acid?

$A$ pungent smelling gas $A$ gives dense white fumes with conc. $HCl$. When $A$ reacts with an alkaline solution of a colourless reagent $B$,a reddish-brown precipitate $C$ is formed. The compounds $A, B, C$ respectively are: