Answer:
Sr^+ is larger than the Mg^2+ ion
Explanation:
As you move down a group in the periodic table, additional shells of electrons are added, these additional shells will normally cause the ionic radius to increase as you move down the periodic table due to greater inter-electronic repulsion. Hence, from the electronic configuration of Mg2+ ( [He]) and that of Sr2+ ([Kr]), the Sr^2+ ion is much larger and has many more intervening shells than Mg2+.
Hence it follows that as we move down group 2, we expect that the ionic radius should increase in accordance with the reason given above. This is exactly the case as shown by the image attached to this answer.
