Which alkaline earth metal has the smallest atomic radius? 

• A. Ca
• B. Rb
• C. Na
• D. Ra
• E. Fr

Answer: (A)

Verified step by step guidance

1. Identify the group of elements known as alkaline earth metals. These are found in Group 2 of the periodic table and include elements like Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), and Radium (Ra).
2. Understand that atomic radius generally increases as you move down a group in the periodic table. This is because each successive element has an additional electron shell, making the atom larger.
3. Examine the list of elements provided in the problem: Ca, Rb, Na, Ra, Fr. Note that not all of these are alkaline earth metals. Calcium (Ca) and Radium (Ra) are the only alkaline earth metals in this list.
4. Compare the atomic radii of the alkaline earth metals identified: Calcium (Ca) and Radium (Ra). Since atomic radius increases down a group, Radium (Ra) will have a larger atomic radius than Calcium (Ca).
5. Conclude that among the alkaline earth metals listed, Calcium (Ca) has the smallest atomic radius.