Arrange the elements $N$,$P$,$O$,and $S$ in the order of:
$(i)$ Increasing first ionisation enthalpy.
$(ii)$ Increasing non-metallic character.
Give reasons for the arrangement assigned.

(N/A) The positions of the elements in the periodic table are as follows:

Period	Group $15$ and $16$
$2$nd period	$N$ (Group $15$),$O$ (Group $16$)
$3$rd period	$P$ (Group $15$),$S$ (Group $16$)

$(i)$ Ionisation enthalpy of nitrogen $(N: 1s^2, 2s^2, 2p^3)$ is greater than oxygen $(O: 1s^2, 2s^2, 2p^4)$ due to the extra stability of the exactly half-filled $2p$-orbitals. Similarly,the ionisation enthalpy of phosphorus $(P: 1s^2, 2s^2, 2p^6, 3s^2, 3p^3)$ is greater than sulphur $(S: 1s^2, 2s^2, 2p^6, 3s^2, 3p^4)$.
On moving down a group,ionisation enthalpy decreases due to an increase in atomic size. Therefore,the order of increasing first ionisation enthalpy is: $S < P < O < N$.
$(ii)$ Non-metallic character increases across a period (left to right) and decreases down a group. Therefore,the order of increasing non-metallic character is: $P < S < N < O$.