Question

Identify the alkali metal having the smallest atomic size from following.

• A. Sodium
• B. Potassium
• C. Rubidium
• D. Caesium

Hint:

Atomic size increases down a group, so pick the highest element listed.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Down a group, new electron shells are added. So atomic size grows from top to bottom in alkali metals: Li < Na < K < Rb < Cs.

Step 2: Apply to the options:
Among sodium, potassium, rubidium and caesium, sodium is the highest in the group and has the fewest shells (three). Its atomic radius is about 186 pm, against about 227 pm for K, 248 pm for Rb and 265 pm for Cs.

Step 3: Conclusion:
So sodium has the smallest atomic size in this list, option (A). The other three have more shells, so they are larger.

Final Answer:
Sodium is the topmost element in the list, so it is the smallest. \[ \boxed{A:\ \text{Sodium}} \]