The first ionization enthalpies () of the Group 13 elements exhibit an irregular trend due to transi — Periodic Table and Periodicity Chemistry Question
Question
The first ionization enthalpies ($IE_1$) of the Group 13 elements exhibit an irregular trend due to transition and lanthanide contractions. Which of the following represents the CORRECT increasing order of $IE_1$ for these elements?
Answer: C
💡 Solution & Explanation
The expected group trend is a continuous decrease down the group. However, poor shielding by $3d$ electrons (transition contraction) makes $Ga > Al$, and poor shielding by $4f$ electrons (lanthanide contraction) makes $Tl > In$. The overall actual increasing order is $In < Al < Ga < Tl < B$.
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