Question

order the elements Mg, K, Ca, S according to second ionization energy

I know that K would have the highest second ionization energy, then come S bec it takes more energy to remove an electron from a nonmetal than a metal, which is next ?

Answer 1

Your first two choices are correct.

Removing the second electron from both Mg and Ca is relatively easy because it is a single s electron in an outer shell, but Ca+ will be easier to ionize because the electron is farther from the nucleus than it is for Mg+.