Which of the following pairs of elements would you expect to have lower first ionisation energy?
a.`Cl` or `F`
b.`Cl` or `S`
c.`K` or `Ar`
d.`Kr` or `Xe`

Explanation

(i) Cl is expected to have lower first ionization enthalpy than F because the ionization enthalpy decreases down the group.
(ii) S is expected to have lower ionization enthalpy that Cl because ionization enthalpy increases along a period from left to right.
(iii) K is expected to have lower ionization enthalpy as compared to Ar. The high value of ionization enthalpy of Ar is due to its completely filled configuration.
(iv) Xe is expected to have lower ionization enthalpy because ionization enthalpy decreases down the group and Xe lies below Kr.