Question

why 1st ionisation energy of magnesium is higher and 2nd ionisation energy is lower than that of sodium.​

Answer 1

The more stable the electron is in the atom, the more difficult it is to remove from the atom and the higher the ionization energy is. The valence electrons in magnesium are more stable than sodium mainly because magnesium has one more proton in the nucleus than sodium does.

Answer 2

The more stable the electron is in the atom, the more difficult it is to remove from the atom and the higher the ionization energy is.
The valence electrons in magnesium are more stable than sodium mainly because magnesium has one more proton in the nucleus than sodium
does

Same as above
Definitely agree