The radius of calcium is smaller than potassium because both elements lie in the third period of the periodic table. Potassium lies to the left of calcium in the period, and atomic size decreases, going left to right due to increase in the force of attraction by the nucleus.
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❥A᭄ɴsᴡᴇʀ࿐:
(D) option
Atomic radius is increases down the Group and decreases across the period.
The radius of calcium is smaller than potassium because both elements lie in the third period of the periodic table. Potassium lies to the left of calcium in the period, and atomic size decreases, going left to right due to increase in the force of attraction by the nucleus.
therefore the correct answer is calcium.