(4)

(A) In group 15, the ionisation enthalpy decreases down the group due to increase in atomic size and screening effect, so N has the highest ionisation enthalpy.

(B) Fluorine is most reactive halogen, it can replace chloride, bromide and iodide ions in solution. In fact, fluorine is so reactive that it attacks water and displaces the oxygen of water. Thus, it is the strongest oxidising agent.

(C) Electronic configuration of ${\mathrm{O}}_2$ is

$\mathrm{\sigma }{\left(1\mathrm{s}\right)}^2 {\mathrm{\sigma }}^{*}{\left(1\mathrm{s}\right)}^2 \mathrm{\sigma }{\left(2\mathrm{s}\right)}^2 {\mathrm{\sigma }}^{*}{\left(2\mathrm{s}\right)}^2 \mathrm{\sigma }{\left(2{\mathrm{p}}_{\mathrm{z}}\right)}^2 \mathrm{\pi }{\left(2{\mathrm{p}}_{\mathrm{x}}\right)}^2=\mathrm{\pi }{\left(2{\mathrm{p}}_{\mathrm{y}}\right)}^2{\pi }^{*}{\left(2{\mathrm{p}}_{\mathrm{x}}\right)}^1={\mathrm{\pi }}^{*}{\left(2{\mathrm{p}}_{\mathrm{y}}\right)}^1$

It has two unpaired electrons, so it is paramagnetic in nature.

(D) Xenon, due to its low ionisation enthalpy, is a most reactive element of group 18 and forms a large number of compounds.

(1)

Helium is used as a diluent for oxygen in modern diving apparatus because of its very low solubility in blood.

(3)

Noble gases have very low melting and boiling points because the only type of interatomic interaction in these elements is weak dispersion forces.