Question

Which of the following factors may be regarded as the main cause of lanthanide contraction?

• A. Poor shielding of one of 4f electron by another in the sub-shell.
• B. Effective shielding of one of 4f electrons by another in the subshell
• C. Poorer shielding of 5d electrons by 4f electrons
• D. Greater shielding of 5d electrons by 4f electrons

Hint:

The 4f electrons shield one another poorly.

Answer 1

The Correct Option is A

Step 1: Reasoning:
Atomic size depends on the balance between nuclear pull and electron shielding.

Step 2: Across the series:
Nuclear charge goes up by one each element. The new electron enters $4f$, which shields badly. So net pull on the outer electrons grows.

Step 3: Result:
Radius shrinks. The cause is the weak mutual shielding of $4f$ electrons, option (A).

Final Answer:
Nuclear charge rises but 4f electrons shield each other badly. \[ \boxed{A} \]