Question

Second ionization enthalpy of Na is higher than the Mg why?

Answer 1

The first ionization of Na results in Na+ having a full valence shell similar to Ne. The second ionization will then be to remove an additional electron so that it will not have a full valence shell. On the other hand, for Mg, the first ionization will result in a valence similar to that of Na. The second ionization will result in a valence similar to Ne. Since having a full valence shell is very favorable, we can guess that I2 of Mg will be lower than I2 of Na.