How would you explain the fact that the first ionization enthalpy of sodium is lower than that of magnesium but its second ionization enthalpy is higher than that of magnesium?

Na and Mg have electronic configurations [Ne]3s¹ and [Ne]3s² respectively. Since Mg has a completely filled s orbital than Na, the electronic configuration of Mg is more stable than that of Na. Hence, Mg has higher first ionization energy than Na. When an electron is lost from Na, it acquires the stable electronic configuration of noble gas Ne. When Mg loses one electron, it acquires the electronic configuration [Ne]3s¹. Hence, Na⁺ has a more stable electronic configuration than Mg⁺. Thus, Na has a much larger second ionization enthalpy than Mg.