Why does $R_{3}P=O$ exist but $R_{3}N=O$ does not ($R=$ alkyl group)?

Which of the following order is incorrect?

$A$ 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 $.......$

Among $P_2O_5$,$As_2O_3$,$Sb_2O_3$,and $Bi_2O_3$,the most acidic oxide is

Which of the following statements is correct?

$Pb + \text{Dil. } HNO_3 \xrightarrow{\text{Warm}} P + Q\uparrow + H_2O$
Incorrect statement for $Q$ is