Actinoid contraction is greater from element to element than Lanthanoid contraction because of

poor shielding by 5f electrons

The correct answer is Option (4) → poor shielding by 5f electrons

Actinoid contraction refers to the gradual decrease in atomic and ionic radii of actinoid elements with increasing atomic number.

This contraction is greater than lanthanoid contraction.

The main reason is the very poor shielding ability of 5f electrons.

As electrons are added to the 5f orbitals, they do not effectively shield the increasing nuclear charge. As a result, the effective nuclear charge increases significantly, pulling electrons closer to the nucleus and decreasing atomic size more rapidly.

Thus, the contraction from element to element becomes larger in the actinoid series.

Option A: Poor shielding by 4f electrons

This explains lanthanoid contraction, not actinoid contraction. Incorrect

Option B: Prior filling of 4d electrons

4d orbitals are filled in transition metals before lanthanoids, not relevant to actinoid contraction. Incorrect

Option C: Prior filling of 3d electrons

3d orbitals belong to the first transition series and are unrelated to actinoid contraction. Incorrect

Option D: Poor shielding by 5f electrons

5f electrons shield the nuclear charge very poorly, leading to stronger contraction in actinoids. Correct.