Question

Which one of the following has the lowest ionization enthalpies ? (a) 4s^1 (b) 3d^2 (c) 3p^6 (d) 2p^6

Answer 1

4s1 is the outermost shell among all these and thus the least effective nuclear charge acts on it. You can also verify it by the fact that Potassium which has it valence electron in 4s1 is a highly positive metal.