Which of the following oxyacids contains both $P-H$ and $P-P$ bonds simultaneously?

$A$ group $15$ element forms $d \pi - d \pi$ bond with transition metals. It also forms a hydride,which is the strongest base among the hydrides of other group members that form $d \pi - d \pi$ bonds. The atomic number of the element is $.......$

Which of the following molecules contains the highest number of $P-H$ bonds?

What happens when a large amount of water is added to a solution of $BiCl_3$?

Nitrogen is liberated by the thermal decomposition of only:

Discuss the trends in chemical reactivity of group $15$ elements.