Question

Arrange the elements N, P O and S in the order of
(i) increasing first ionisation enthalpy.

(ii) Increasing non-metalic character. Give reason for the arrangement assigned.

Answer 1

The placing of elements are as
$$\begin{array}{ccc}Period& Group15& Group16\\ 2ndperiod& N& O\\ 3rdperiod& P& S\end{array}$$
Ionisation enthalpy of nitrogen ${(}_{7}N=1s^2,2s^2,2p^3)$ is greater than oxygen ${(}_{8}O=1s^2,2s^2,2p^4)$ due to extra stable exactly half - filled 2p - orbitals. Similarly, ionisation enthalpy of phosphorus${(}_{15}P=1s^2,2s^2,2p^6,3s^2,3p^3)$ is greater than sulphur ${(}_{18}S=1s^2,2s^2,2p^6,3s^2,3p^4)$
On moving down the group, ionisation enthalpy decreases with increasing atomic size. So the orter is
$S<P<O<N\to$

In another way, Ionisation enthalpy increases from left to right in a period and decreases down the group. N has higher ionisation enthalpy than O due to the extra stability of half-filled orbitals. Similarly, P has higher ionisation enthalpy than S due to half-filled orbitals

(ii) Non - metallic character across a period (left to right ) increases but on moving down the group it decreases, So, the order is
$P<S<N<O\to$ Non-metallic character increases