Question

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

Answer 1

Electronic configuration of \[\ce{Na}\] is 1s²2s²2p⁶3s¹. After losing one electron from its outermost shell, sodium easily attains stable electronic configuration (1s²2s²2p⁶ ), while magnesium does not lose its electron easily due to presence of two electrons in s-orbital (1s²2s²2p⁶3s²). Hence first ionisation energy of sodium is less than magnesium.

When one electron is removed from \[\ce{Na}\] and \[\ce{Mg}\], their configurations become ls²2s²2p⁶ and 1s²2s²2p⁶3s¹ respectively. Now it is easier to remove one electron from 3s of \[\ce{Mg+}\] than 2p⁶ of \[\ce{Na+}\]. Hence, second ionisation energy of \[\ce{Mg}\] is less than \[\ce{Na}\].