NCERT Exercise 3.19 - Classification of Elements and Periodicity in Properties
Question: Explain the irregular Group 13 first ionisation enthalpies B 801, Al 577, Ga 579, In 558 and Tl 589 kJ mol^-1.
Answer: Poor shielding by d/f electrons increases effective nuclear charge: Ga is slightly above Al, and Tl is above In due to lanthanoid contraction and inert-pair effects.
Full step-by-step NCERT solution with diagrams on Something by Zero.