Slatters Rule#!#Ionisation Enthalpy

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

The correct order of Ionisation Enthalpy is -

Why second ionization enthalpy is always higher than the first ionisation enthalpy for every element?

Which of the following ioelectronic ions have the lowest ionisation enthalpy ?

Periodicity In Ionisation Enthalpy

The second ionisation enthalpies of elements are always higher than their first ionisation enthalpies because: