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(b) Metallic radius.

Explanation

Atomic radius refer to both metallic radius and covalent radius.
(b) Crystal radius (metallic radius) : It is defined as one-half of the inter-nuclear distance between two adjacent atoms of a metal.
Ex. Crystal radius of sodium = `(3.72)/2 = 1.86` Å
Crystal radius of potassium = `(4.62)/(2) = 2.31` Å.