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Problem 6 - Entrance Test

Which of the following statements about the lanthanide contraction is INCORRECT? (A) It is primarily caused by the poor shielding of the 4f electrons, which results in a greater effective nuclear charge across the series. (B) It leads to an increase in the ionic radii of the Ln³⁺ ions from La³⁺ to Lu³⁺. (C) It causes the radii of the 4d and 5d transition metals to be very similar, making them harder to separate. (D) It results in the basicity of the lanthanide hydroxides decreasing from La(OH)₃ to Lu(OH)₃.

Correct: B

The lanthanide contraction is caused by the poor shielding of 4f electrons, leading to a gradual increase in effective nuclear charge and a decrease in ionic radii across the series (from La³⁺ to Lu³⁺). Statement B is incorrect because the ionic radii decrease, not increase, from La³⁺ to Lu³⁺. Statement C is correct: the contraction causes 4d and 5d elements to have similar sizes. Statement D is correct: basicity decreases across the series as ionic radius decreases. Statement A correctly describes the cause. Thus, the INCORRECT statement is B.