Question

which of the following has the lowest ionisation energy...K+,Ca2+,Cl-,S2-

Answer 1

if we are starting w/ ions as indicated then note they are all isoelectronic w/ Ar - thus the one with themost protons will have greatest ionication energy 

P-3 < S -2 < Cl- < K+ < Ca+2......

Answer 2

Answer:  $K^+$

Explanation:

The ionization energy is the energy required to remove an electron from the valence shell of an isolated gaseous atom of an element.

Now the ionization energy depends upon

a) size of an element ,larger the size more lesser the effective nuclear charge on electron thus lower the ionization energy.

b) the electronic configuration of an element. If on losing an electron an element attains noble gas configuration or half filled stability then it will easily lose an electron.

$K^+:1s^22s^22p^63s^1$$Ca^{2+}:1s^22s^22p^63s^23p^6$

$Cl^-:1s^22s^22p^63s^23p^6$

$S^{2-}:1s^22s^22p^63s^23p^6$

In case of potassium, if it loses an electron it will attain the noble gas configuration of Neon.  

So it is most easy to remove electron from $K^+$ as all other species already have stable configurations.