Question

Reason of lanthanoid contraction is:

• A. Increasing nuclear charge
• B. Decreasing nuclear charge
• C. Decreasing screening effect
• D. Negligible screening effect of 'f' orbitals

Answer 1

The Correct Option is D

The lanthanoid contraction refers to the phenomenon where there is a gradual decrease in the ionic and atomic radii of the lanthanide series elements (from La to Lu) despite having an increasing atomic number. The primary reason for this contraction is the negligible screening effect of the 'f' orbitals.

Here's a step-by-step explanation:

1. Understanding Electron Screening: The screening effect is the ability of inner electron shells to shield outer electrons from the attractive force of the nucleus. The effectiveness of this shielding decreases as the subshell's type changes from s to p to d and is least effective in f orbitals.
2. Role of 'f' Orbitals: In lanthanides, electrons are progressively added to the 4f subshell. The 4f electrons are poorly effective in screening (or shielding) the outer electrons from the increasing nuclear charge. As a result, the outer electrons experience a stronger pull from the nucleus.
3. Nuclear Charge and Atomic Radius: As the nuclear charge increases with each successive element in the series, the inadequately shielded outer electrons are pulled closer to the nucleus, leading to a decrease in atomic and ionic sizes.

Among the given options:

• Increasing nuclear charge: This is true but not the reason for the contraction; it contributes to why the contraction happens alongside poor screening.
• Decreasing nuclear charge: Incorrect, as the nuclear charge actually increases across the series.
• Decreasing screening effect: Incorrect as the screening effect by the 'f' orbitals is inherently poor; it does not decrease but is insufficient from the start.
• Negligible screening effect of 'f' orbitals: This is the correct reason. The poor shielding effect of f orbitals allows the nuclear charge to pull the electrons closer, causing the contraction.

Thus, the correct answer is that the reason for lanthanoid contraction is the negligible screening effect of 'f' orbitals.