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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?
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Solution
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 \[\ce{Na}\].
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