Which member in each of the following pairs has lower value of ionisation potential
(1) F, Cl
(2) S, Cl
(3) Ar, K.

Explanation

(1) Cl has lower ionisation enthalpy than F because electrons of 2p-orbital in F are more strongly attracted by the nucleus than the 3p-electrons in Cl, (Note that effective nuclear charge on the outermost electrons is nearly the same for both F and Cl).
(2) S has lower ionisation enthalpy than Cl because the size of S is greater than that of Cl and also the nuclear charge of S is less thant that of Cl.
(3) K has lower ionisation potential than Ar as outermost shell is completely filled with electrons in Ar. on the other hand K can attain a stable configuration like the inert gas Ar by the loss of only 1 electron.