Question

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

Answer 1

Na and Mg have electronic configurations $\left[Ne\right]3s^1$ and $\left[Ne\right]3s^2$ respectively. Since, Mg has 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 stable electronic configuration of noble gas Ne. When Mg loses one electron, it acquires the electronic configuration $\left[Ne\right]3s^1$.

Hence, $N{a}^{+}$ has more stable electronic configuration than $M{g}^{+}$. Thus, Na has much larger second ionization enthalpy than Mg.