Consider the elements Ne , Na , Mg . The element with highest first ionization enthalpy and element with lowest second ionization enthalpy respectively are

Step-by-step solution

aligned & ₁₀ Ne =1 ~s ^2 2 ~s ^2 2 p ^6, ₁₁ Na =1 ~s ^2 2 ~s ^2 2 p ^6 3 ~s ^1, & ₁₂ Mg =1 ~s ^2 2 ~s ^2 2 p ^6 3 ~s ^2 aligned Highest first ionization enthalpy = Ne , due to stable fully-filled configuration. Lowest second ionization enthalpy = Mg , due to stable noble - gas configuration obtained after losing two electrons.