Which of the following atoms has the smallest atomic radius?

• A. Na
• B. Mg
• C. Cl
• D. K

Answer: (C)

Verified step by step guidance

1. Recall that atomic radius generally decreases across a period (left to right) in the periodic table and increases down a group (top to bottom).
2. Identify the positions of the given elements in the periodic table: Na (Sodium) and Mg (Magnesium) are in period 3, group 1 and 2 respectively; Cl (Chlorine) is in period 3, group 17; K (Potassium) is in period 4, group 1.
3. Since Na, Mg, and Cl are all in the same period (period 3), compare their atomic numbers and effective nuclear charge. As you move from left to right across a period, the number of protons increases, pulling electrons closer and decreasing atomic radius.
4. K is in the period below (period 4), so it has a larger atomic radius due to the addition of an electron shell.
5. Therefore, among the options, Cl has the smallest atomic radius because it is furthest to the right in period 3, meaning it has the highest effective nuclear charge and pulls its electrons closer.