Question

The steady, progressive decrease in the atomic and ionic radii of transition metals along the lanthanide series (Lanthanide Contraction) is primarily caused by:

• A. A continuous decrease in the absolute nuclear proton count within the central atomic cores.
• B. The exceptionally poor shielding effect of the diffuse \(4f\)-electrons, which allows the increasing nuclear charge to pull the outer electron shell inward.
• C. The strong shielding ability of the \(5d\)-electrons completely blocking the nuclear pull.
• D. A sudden jump in the principal quantum number of the outer valence shells.

Hint:

Due to lanthanide contraction, elements of the \(4d\) and \(5d\) transition series in the same group often have very similar atomic radii and chemical properties, such as Zr and Hf.

Answer 1

The Correct Option is B

Concept: Lanthanide contraction refers to the gradual decrease in atomic and ionic radii across the lanthanide series from La to Lu. This occurs because electrons are progressively added to the \(4f\)-subshell while the nuclear charge also increases.

Step 1: Understand the shielding effect of \(4f\)-electrons.
As we move across the lanthanide series:

• One proton is added to the nucleus at each step.
• One electron is added to the \(4f\)-orbital.

However, \(4f\)-electrons have very poor shielding ability because their orbitals are highly diffuse and do not effectively block the nuclear attraction experienced by outer electrons. The shielding effectiveness follows: \[ s > p > d > f \] Thus, \(f\)-electrons are the least effective in shielding.

Step 2: Analyze the effect on atomic size.
Because the \(4f\)-electrons cannot effectively shield the increasing nuclear charge: \[ Z_{\text{eff}} \uparrow \] where \(Z_{\text{eff}}\) represents effective nuclear charge. As the effective nuclear charge increases, the outer electrons are pulled closer to the nucleus. Therefore: \[ \text{Atomic Radius} \downarrow \] This gradual decrease in size across the lanthanide series is called lanthanide contraction.

Step 3: Evaluate the options.

• [(A)] Incorrect — Nuclear proton count increases across the series, it does not decrease.
• [(B)] Correct — Poor shielding by \(4f\)-electrons causes increasing effective nuclear charge and contraction in size.
• [(C)] Incorrect — \(5d\)-electrons are not responsible for lanthanide contraction.
• [(D)] Incorrect — There is no sudden jump in the principal quantum number causing this effect.

Hence, the correct answer is: \[ \boxed{\text{(B)}} \]