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Question: The ionisation energies for $B. Tl$ and In are X.Y and ZK cal/mol respectively. Choose the correct r...

The ionisation energies for B.TlB. Tl and In are X.Y and ZK cal/mol respectively. Choose the correct relationship between them-

A

Z > X = Y

B

X > Y > Z

C

X < Y < Z

D

X < Y < > Z

Answer

B

Explanation

Solution

The ionization energy generally decreases down a group due to increasing atomic size and shielding. However, Group 13 exhibits exceptions.

  • Boron (B) has the smallest size and highest effective nuclear charge, thus the highest ionization energy (X).

  • Thallium (Tl) has an anomalously high ionization energy (Y) compared to Indium (In) (Z) due to the poor shielding by 4f and 5d electrons (lanthanide contraction and relativistic effects), leading to an increased effective nuclear charge. Therefore, IE(Tl) > IE(In), i.e., Y > Z.

Despite this anomaly, Boron's ionization energy remains the highest in the group due to its extremely small size and strong nuclear attraction. Thus, IE(B) > IE(Tl), i.e., X > Y. Combining these, the correct relationship is X > Y > Z.