The atomicity of phosphorus is $X$ and the $P-P-P$ bond angle in the molecule is $Y$. What are $X$ and $Y$?

Silver nitrate is prepared by the reaction of:

Account for the following observations:
$(A)$ $AlCl_3$ is a Lewis acid.
$(B)$ Though fluorine is more electronegative than chlorine yet $BF_3$ is a weaker Lewis acid than $BCl_3$.
$(C)$ $PbO_2$ is a stronger oxidizing agent than $SnO_2$.
$(D)$ The $+1$ oxidation state of thallium is more stable than its $+3$ state.

Which of the following statements is true for $PH_3$?

Nitrogen exists as $N_2$,whereas phosphorus exists as $P_4$. This is because ..........

Heating $(NH_4)_2Cr_2O_7$ liberates a gas. The same gas is obtained by: