Question

Reason of lanthanoid contraction is

• A. negligible screening effect of 'f'-orbitals
• B. increasing nuclear charge
• C. decreasing nuclear charge
• D. decreasing screening effect

Answer 1

The Correct Option is A

The question asks for the reason behind the lanthanoid contraction, which is a consistent decrease in atomic and ionic radii of lanthanide elements (elements 57-71) as they increase in atomic number.

1. Lanthanoid Contraction Definition: As you move across the lanthanide series from lanthanum to lutetium, the addition of electrons to the f-orbitals results in a gradual decrease in atomic and ionic size despite an increase in atomic number.
2. Screening Effect: The screening effect (or shielding effect) refers to the ability of inner electrons to shield the outer electrons from the full effect of the nucleus's positive charge.
3. 'f'-orbitals and Screening: The f-orbitals are diffused in nature and do not shield the nuclear charge effectively. This results in a very negligible screening effect by these orbitals.
4. Nuclear Charge Influence: Although the nuclear charge increases across the series, the poor shielding by f-electrons means that the electrons are drawn closer to the nucleus more effectively, leading to a contraction in size.
5. Correct Reason: The negligible screening effect of 'f'-orbitals is primarily responsible for the lanthanoid contraction. As the f-orbitals do not shield well, the effective nuclear charge experienced by the electrons increases, causing contraction.

Conclusively, the correct answer is negligible screening effect of 'f'-orbitals. Other options are ruled out because they do not accurately explain the effect as seen in the lanthanoids:

• Increasing nuclear charge - while true, it does not address the unique aspect of poor f-orbital shielding.
• Decreasing nuclear charge - incorrect, the nuclear charge actually increases.
• Decreasing screening effect - not specific to f-orbital characteristics, which are critical here.