Question

Why the first ionisation enthalpy of sodium is lower than that of magnesium while
its second ionisation enthalpy is higher than that of magnesium?
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Answer 1

Answer:

Hence, in case of sodium, the second electron has to be removed from the stable noble gas configuration. Consequently, removal of a second electron from sodium requires more energy in comparison to that required in magnesium. Therefore, the second ionization enthalpy of sodium is higher than that of magnesium.

Step-by-step explanation:

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Answer 2

Answer:

Sodium has only one electron extra to achieve stable noble gas configuration (electronic configuration of Sodium 1s2 2s2 2p6 3s1), hence it easily loses one electron. But when we consider its second ionisation enthalpy it is very high as it has to lose its stable configuration by losing one electron. On the other hand Mg doesn't face such stability issues.