$(D)$ The atomic radius of $Al$ is $143 \ pm$ and that of $Ga$ is $135 \ pm$.Thus, the atomic radius of $Ga$ is actually smaller than that of $Al$.Thi

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If both the Assertion and Reason are incorrect.

$(D)$ The atomic radius of $Al$ is $143 \ pm$ and that of $Ga$ is $135 \ pm$.Thus, the atomic radius of $Ga$ is actually smaller than that of $Al$.This occurs because, in $Ga$, the $3d^{10}$ electrons provide poor shielding of the nuclear charge, leading to a higher effective nuclear charge which pulls the valence shell closer to the nucleus.Since the Assertion is false and the Reason is true, the correct option is $D$.

Class 11 Chemistry Classification of Elements and Periodicity in Properties Atomic and Ionic radii

Difficult

Assertion : Atomic radius of gallium is higher than that of aluminium.
Reason : The presence of additional $d$-electrons offers poor screening effect for the outer electrons from increased nuclear charge.

AIIMS 2017 All AIIMS

A
If the Assertion is correct but Reason is incorrect.
B
If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
C
If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
D
If both the Assertion and Reason are incorrect.