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?
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?

When sodium loses an electron from its outermost shell, it achieves a stable state. As a result, its initial ionization enthalpy is lower than that of magnesium. However, in the case of second ionization enthalpy, magnesium has one electron in its outermost shell, which it loses quickly in order to achieve stability, but sodium is already stable.