Which of the following elements has the greatest atomic radius?

• A. Mg
• B. K
• C. Na
• D. Al

Answer: (B)

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. Recall that atomic radius generally decreases as you move from left to right across a period (row) in the periodic table because the increasing nuclear charge pulls electrons closer to the nucleus.
3. Identify the positions of the elements in the periodic table: Mg (magnesium) is in period 3, group 2; Na (sodium) is in period 3, group 1; Al (aluminum) is in period 3, group 13; K (potassium) is in period 4, group 1.
4. Compare the elements by considering both their group and period: K is below Na in group 1, so it has an additional electron shell compared to Na, making K larger than Na. Mg and Al are in period 3 but to the right of Na, so they have smaller atomic radii.
5. Conclude that potassium (K), being in period 4 and group 1, has the greatest atomic radius among the given elements due to its position further down the group and to the left in the periodic table.