NCERT Exercise 3.17 - Classification of Elements and Periodicity in Properties
Question: Why is first ionisation enthalpy of Na lower than Mg but second ionisation enthalpy of Na higher than Mg?
Answer: Na loses its 3s electron more easily first; after that Na+ is neon-like, so removing a core electron is much harder than removing Mg+'s remaining 3s electron.
Full step-by-step NCERT solution with diagrams on Something by Zero.