Which of the following isoelectronic ions has the lowest ionization energy?

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 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. Hence the order of their relative sizes will be 2 2 S Cl K Ca . (Isolectronic) Therefore, 2 S will have the lowest