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

On heating lead $(II)$ nitrate gives a brown gas $'A'$. The gas $'A'$ on cooling changes to a colourless solid $'B'$. Solid $'B'$ on heating with $NO$ changes to a blue solid $'C'$. Identify $'A'$,$'B'$ and $'C'$,write the reactions involved,and draw the structures of $'B'$ and $'C'$.

$P_{4}O_{10}$ reacts with water to produce

Which among the group $15$ elements does $NOT$ exist as a tetra-atomic molecule?

Assertion : Molecular nitrogen is less reactive than molecular oxygen.
Reason : The bond length of $N_2$ is shorter than that of oxygen.

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