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

Poor shielding of one of 4f electrons by another in the subshell

The correct answer is option 1. Poor shielding of one of 4f electrons by another in the subshell.

Lanthanide contraction refers to the phenomenon observed across the lanthanide series of elements where there is a relatively small increase in atomic and ionic radii as you move from lanthanum (La) to lutetium (Lu). This is in contrast to the predictable increase in atomic size seen in other periods of the periodic table.

Causes of Lanthanide Contraction: The main cause of lanthanide contraction can be attributed to the poor shielding of one 4f electron by another within the same subshell. Here's a detailed explanation:

1. 4f Electron Shielding: The lanthanides have their valence electrons filling the 4f subshell. As you move across the lanthanide series, additional electrons are added to the 4f subshell.

2. Ineffective Shielding: Despite the addition of electrons to the 4f subshell, these electrons do not effectively shield the increasing nuclear charge from each other. Each new electron added to the 4f subshell experiences a progressively stronger attraction from the positively charged nucleus due to the increasing nuclear charge.

3. High Effective Nuclear Charge: The result is that the effective nuclear charge experienced by the outermost electrons (including the 5d and subsequent orbitals) remains relatively high across the lanthanide series. This high effective nuclear charge leads to a stronger pull on the outer electrons, resulting in a smaller increase in atomic and ionic radii.

4. Consequences: The lanthanide contraction affects various properties of the elements, including their atomic size, ionic size, and chemical behavior. It influences the stability of complexes, the solvation energies of ions, and other chemical and physical properties.

Comparison to Transition Metals

In contrast to lanthanides, transition metals do not experience a significant contraction in atomic size across their series. This is because transition metals fill their 3d subshell, which has better shielding capabilities compared to the lanthanide 4f subshell.

Summary

Lanthanide contraction is primarily caused by the poor shielding of one 4f electron by another within the same subshell. This ineffective shielding leads to a smaller increase in atomic and ionic radii across the lanthanide series than expected based on periodic trends. Understanding lanthanide contraction is essential in chemistry and materials science for predicting and explaining the properties of lanthanide elements and their compounds.