Final answer:
Based on periodic trends, calcium (Ca) has a larger atomic radius than magnesium (Mg) because it is located below magnesium in the periodic table, indicating an increase in the principal quantum number and an additional electron shell.
Step-by-step explanation:
When comparing the atomic radii of calcium (Ca) and magnesium (Mg), we can rely on periodic trends to determine which element has the larger radius. Both calcium and magnesium are located in the same group in the periodic table, Group 2, which means they are alkaline earth metals. However, calcium is positioned below magnesium, which implies it has a larger atomic radius due to an increase in the principal quantum number, n. As we move down a group in the periodic table, each element has an additional electron shell compared to the element above it, increasing the atomic radius.
Considering the periodic trend that atomic radii increase down a group, we can conclude that calcium has a larger atomic radius than magnesium. Furthermore, as cations, we can also say that cations with larger charges are smaller, so their ionic radii would also reflect this trend if they had differing charges. However, both Ca2+ and Mg2+ have a charge of +2, so the comparison is straightforward with calcium maintaining the larger radius in its ionic form as well.
Therefore, based on the periodic trends and the considerations of atomic and ionic radii, the answer to the question, "What is the calcium vs magnesium radius?" is A) Calcium has a larger radius.