Which of the following has the smallest atomic radius: mg2+, mg, ca2+, or ca?

The appropriate response is Mg2+. It has the smallest radius the would mean the littlest separation from it's the furthest shell to the core 
Since magnesium has one less shell that calcium, Mg would have a little nuclear range. Besides, a particle of a component will have a little nuclear sweep than it's molecule be utilized as a part of its particle frame it has lost a shell