Which of the following isoelectronic ion has the lowest ionization energy
(a)K+
(b)Cl-
(c)Ca2+
(d) S2-

Dear student,

The electronic configuration of all the ions are: $1s^2 2s^2 2p^6 3s^2 3p^6$
When we move left to right in the periodic table the ionization energy increases and when we move up to down in the group the ionization energy decreases.
As K(potassium) from the extreme left of the periodic table in group 1 will have the lowest ionization energy.

Regards