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Assertion (A): The decrease in atomic radius per element in the actinide series is larger than the corresponding decrease in the lanthanide series. Reason (R): The 5f electrons in actinides are poor at shielding the nuclear charge from one element to the next within the series.

1. A is false but R is true.
2. A is true but R is false.
3. Both A and R are true and R is the correct explanation of A.
4. Both A and R are true and R is NOT the correct explanation of A.

Correct answer: Both A and R are true and R is the correct explanation of A.

Solution

Actinide contraction (per element step) is indeed greater than lanthanide contraction because 5f electrons shield nuclear charge even more poorly than 4f electrons. Poor 5f shielding means each added proton has a greater net pull on the outer electrons, causing a larger reduction in radius. Thus R correctly explains A.

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