Which of the following statements are correct?

A $5 f$ electrons are more effective than $4 f$ electrons in shielding nuclear charge
BThe ionic radii of tripositive actinoid ions decrease on moving from Ac to Lr
CThe contraction from element to element is smaller in actinoids than in lanthanoids
D $5 f$ electrons shield the nuclear charge poorly, producing actinoid contraction

Answer: B, D

Poor shielding by the diffuse

5 f

subshell raises Zeff steadily and shrinks the

M X^{3 +}

ions along the series. The

5 f

electrons shield worse than

4 f

, and as a result the actinoid contraction is larger, not smaller, than the lanthanoid contraction.