MHT CET Chemistry Question (2026) — Solution

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In the actinoid series, as the atomic number increases from Actinium (Ac) to Lawrencium (Lr), there is a gradual decrease in the size of atoms or M^ 3+ ions. This phenomenon is known as actinoid contraction, which is due to the poor shielding effect of the 5f electrons. The given elements belong to the actinoid series with the following atomic numbers: Pa (Z = 91) Cm (Z = 96) Fm (Z = 100) Lr (Z = 103) Since Lawrencium (Lr) has the highest atomic number among the given options, it experiences the maximum actinoid contraction. Therefore, Lr^ 3+ has the smallest ionic size. Answer: Lr