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JEE Main2026Jan 28, Shift 1Chemistry
Q.

In period 4 of the periodic table, the elements with highest and lowest atomic radii are respectively.

  1. A

    Na & Cl

  2. B

    K & Se

  3. C

    K & Br

  4. D

    Rb & Br

Solution

Period 4 runs from K (Z = 19) to Kr (Z = 36). Across a period, atomic radius decreases from left to right because effective nuclear charge increases while electrons are added to the same shell.

Na and Cl are in period 3 (not period 4); Rb is in period 5. The alkali metal of period 4 is K (largest), and just before the noble gas, the halogen Br has the smallest atomic radius among the main-group representative elements in period 4 (noble gas Kr is not considered in atomic-radius comparison the same way).

K and Br. Option (3).

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