Ca would be 1st in the list (i.e. smallest ionization energy). For the elements Ca, V, Mn, Ni, Zn which are in period 4 of the periodic table, the outermost electron is in the 4s shell. The effective nuclear charge and attraction for the outermost electron increases across the period, and thus the ionization energy increases in the order: Ca < V < Mn < Ni < Zn Solution Ca would be 1st in the list (i.e. smallest ionization energy). For the elements Ca, V, Mn, Ni, Zn which are in period 4 of the periodic table, the outermost electron is in the 4s shell. The effective nuclear charge and attraction for the outermost electron increases across the period, and thus the ionization energy increases in the order: Ca < V < Mn < Ni < Zn.