JEE Main 2025 Apr 4 Shift 1, Chemistry Q15: Periodicity In Atomic Properties

JEE Main2025Apr 4, Shift 1Chemistry

Given below are the pairs of group 13 elements showing their relation in terms of atomic radius.

$(B < A l)$ , $(A l < G a)$ , $(G a < I n)$ and $(I n < T l)$

Identify the elements present in the incorrect pair and in that pair find out the element (X) that has higher ionic radius ( $M^{3 +}$ ) that the other one. The atomic number of the element (X) is

A
31
B
49
C
13
D
81

Solution

Along group 13, atomic radii are: B $<$ Al $>$ Ga $<$ In $<$ Tl. The anomaly arises at Al vs Ga: gallium is actually slightly smaller than aluminium due to the poor shielding of the intervening 3d electrons (a d-block contraction effect). So the incorrect pair in the list is $(A l < G a)$ .

For the same anomalous reason, the ionic radius of Ga $^{3 +}$ is larger than Al $^{3 +}$ ? No — for the $M^{3 +}$ ions the trend follows the atomic-size pattern, and Ga $^{3 +}$ ( $\sim 62$ pm) is larger than Al $^{3 +}$ ( $\sim 53$ pm).

Element X with the higher ionic radius in this pair is therefore Ga, whose atomic number is 31.

$∴$ Option (1).

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JEE Main 2025 Apr 4 Shift 1, Chemistry Q15: Periodicity In Atomic Properties

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