Which one of the following elements has the smallest atomic radius?

• A. K
• B. Na
• C. Cl
• D. Ar

Answer: (D)

Verified step by step guidance

1. Recall that atomic radius generally decreases from left to right across a period in the periodic table due to increasing effective nuclear charge, which pulls electrons closer to the nucleus.
2. Identify the period and group of each element: K (potassium) and Na (sodium) are alkali metals in Group 1, with K in Period 4 and Na in Period 3; Cl (chlorine) is a halogen in Group 17, Period 3; Ar (argon) is a noble gas in Group 18, Period 3.
3. Compare elements within the same period first: Na, Cl, and Ar are all in Period 3, so their atomic radius decreases from left (Na) to right (Ar) due to increasing nuclear charge without additional shielding.
4. Since Ar is to the right of Cl and Na in the same period, it has a smaller atomic radius than both.
5. K is in Period 4, which generally has a larger atomic radius than Period 3 elements because it has an additional electron shell, so Ar has the smallest atomic radius among the given elements.