Which of the following elements has the largest covalent radius?

• A. Al
• B. Na
• C. K
• D. Mg

Answer: (C)

Verified step by step guidance

1. Recall that the covalent radius of an element generally increases as you move down a group (column) in the periodic table because additional electron shells are added, increasing the size of the atom.
2. Identify the group and period of each element: Na (Sodium) and K (Potassium) are in Group 1 (alkali metals), with Na in Period 3 and K in Period 4; Mg (Magnesium) and Al (Aluminum) are in Period 3 but in Groups 2 and 13 respectively.
3. Understand that elements in the same group have increasing covalent radius down the group, so K (Period 4) will have a larger radius than Na (Period 3).
4. Compare elements across a period: moving from left to right across a period, the covalent radius decreases due to increasing nuclear charge pulling electrons closer, so Mg and Al (both Period 3) will have smaller radii than Na and K.
5. Conclude that among the given elements, K has the largest covalent radius because it is the furthest down the group and to the left in the periodic table, resulting in the largest atomic size.