(A) : The atomic size of gallium is less than expected
(R) : In gallium the 3d¹⁰electrons do not shield effectively

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A is true, but R is false

A and R are true, R does not explain A

A and R are true, R explains A

A is false, but R is true

answer is A.

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Detailed Solution

Gallium has smaller atomic radius than aluminium because the d orbital which is highly diffused offers poor shielding effect which results in increased nuclear charge. Due to increased charge by nucleus, outer electrons get attracted, decreasing the radius.

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