Why a large amount of energy

Why a large amount of energy is needed to remove an electron from a neutral gaseous neon atom than the energy needed to remove an electron from gaseous sodium atom ?

Na(g) + 118.4 k.cal -----> ${{Na}^{+}}$ + ${{e}^{-}}$
Na(g) + 497.0 k;cal -------> ${{Na}^{+}}$ + ${{e}^{-}}$
Orbitals which are fully - filled are very stable, so large amount of energy is needed to remove an electron from them.