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?
Sodium attains stable configuration when it loses on an electron from its outermost shell. That is why its first ionisation enthalpy is less than magnesium. But in case of
second ionisation enthalpy magnesium has one electron in its outermost shell and to attain stability it loses that easily as compared to sodium which is already stable.