Which group-2 element has a slightly different electronic configuration than the rest of the elements?

Explanation

The electrons of Mg having a higher nuclear charge are more strongly attracted towards the nucleus. Thus, the atomic and ionic radii of Mg is less than Na. Again on moving down the group (from Mg to Ca) the atomic and ionic radii increases due to addition of new shells and the increasing screening effect jointly overcome the effect of increasing nuclear charge. Thus, the atomic and ionic radii of Mg is less than Ca.