Which of the following elements has the largest atomic radius?

• A. K
• B. Al
• C. Na
• D. Mg

Answer: (A)

Verified step by step guidance

1. Recall that atomic radius generally increases as you move down a group (column) in the periodic table because additional electron shells are added, making the atom larger.
2. Also remember that atomic radius generally decreases as you move from left to right across a period (row) due to increasing nuclear charge pulling electrons closer to the nucleus.
3. Identify the positions of the elements in the periodic table: K (potassium) is in Group 1, Period 4; Na (sodium) is in Group 1, Period 3; Mg (magnesium) is in Group 2, Period 3; Al (aluminum) is in Group 13, Period 3.
4. Compare the elements by their group and period: since K is in a lower period (Period 4) than Na, Mg, and Al (all Period 3), K has more electron shells, which increases its atomic radius.
5. Conclude that because K is both in Group 1 (which tends to have larger radii) and in a lower period, it has the largest atomic radius among the given elements.