The ionisation enthalpy of `Na` is less than `Ne`. Why?

Explanation

Outermost shell of neon, `Ne(Z=10,1s^(2)2s^(2),2p^(6))` is fully filled, whereas sodium, `Na(Z=11,1s^(2)2s^(2),2p^(6)3s^(1))` has one electron in the valence shell. Hence,`Ne` is more stable than `Na`. Hence ionisation enthalpy of `Na` is less than `Ne`.