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JEE MainChemistryClassification of Elements and Periodicity in Properties

Which of the following correctly represents the relationship between the atomic radii of Aluminium ( Al ) and Gallium ( Ga ) along with the appropriate reason?

Options

  1. Ar_ Ga < r_ Al due to the poor shielding effect of completely filled 3d orbitals in Ga .
  2. Br_ Ga > r_ Al due to the addition of a new principal quantum shell in Ga .
  3. Cr_ Ga < r_ Al due to the lanthanoid contraction experienced by Ga .
  4. Dr_ Ga > r_ Al due to the high screening effect of completely filled 3d orbitals in Ga .

Correct answer

A. r_ Ga < r_ Al due to the poor shielding effect of completely filled 3d orbitals in Ga .

Step-by-step solution

The atomic radius generally increases down a group due to the addition of new electron shells. However, in Group 13, Gallium ( Ga ) has the electronic configuration [ Ar ] 3 d ¹⁰ 4 s ² 4 p ¹ . The completely filled 3d electrons offer poor shielding (or screening) for the outer electrons from the increased nuclear charge. As a result, the effective nuclear charge increases, pulling the valence electrons closer to the nucleus. This causes the atomic radius of Ga (135 pm) to be smaller than that of Al (143 pm). Answer

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