Problem 141 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following factors may be regarded as the main cause of lanthanide contraction? \(\quad\) [2005] (a) poor shielding of one of \(4 \mathrm{f}\) electrons by another in the subshell (b) effective shielding of one of \(4 \mathrm{f}\) electrons by another in the subshell (c) poor shielding of 5 d-electrons by 4 f electrons (d) greater shielding of 5 d-electrons by 4 f electrons

Step-by-Step Solution

Answer

The main cause is (a) poor shielding of one 4f electron by another.

Step 1: Understanding Lanthanide Contraction

Lanthanide contraction refers to the gradual decrease in the size of lanthanide ions from La (Lanthanum) to Lu (Lutetium) as you move across the periodic table. This phenomenon occurs even though the principal quantum number remains constant as the atomic number increases.

Step 2: Analyze the Role of 4f Subshell Shielding

The key concept behind lanthanide contraction is the shielding effect, which is the ability of inner electrons to reduce the effective nuclear charge felt by outer electrons. In the case of lanthanides, the 4f electrons experience poor shielding by other 4f electrons, which means these electrons do not efficiently block the nuclear charge.

Step 3: Evaluate Option (a)

Option (a) states 'poor shielding of one of 4f electrons by another in the subshell'. This correctly reflects the poor shielding effect among 4f electrons, leading to a stronger effective nuclear charge and resulting in the lanthanide contraction.

Step 4: Evaluate Option (b)

Option (b) states 'effective shielding of one of 4f electrons by another in the subshell'. This is incorrect, as it suggests good shielding, which does not align with the cause of lanthanide contraction since good shielding would prevent contraction.

Step 5: Evaluate Option (c)

Option (c) states 'poor shielding of 5 d-electrons by 4 f electrons'. While poor shielding can affect the effective nuclear charge, the primary effect of lanthanide contraction is due to the 4f electrons themselves, not the 5d electrons.

Step 6: Evaluate Option (d)

Option (d) states 'greater shielding of 5 d-electrons by 4 f electrons.' This is incorrect since greater shielding would imply less contraction, contrary to what is observed in lanthanides.

Step 7: Conclusion

The most accurate description of the main cause of lanthanide contraction is outlined in option (a), which highlights the poor shielding among 4f electrons leading to increased effective nuclear charge and the subsequent decrease in ionic radii.

Key Concepts

Shielding Effect

The shielding effect plays a crucial role in understanding why different electrons within an atom do not all feel the same amount of nuclear charge. Essentially, it describes how electrons in the inner shells, closer to the nucleus, can block or reduce the full positive charge of the nucleus from affecting outer electrons in larger orbits.

In atoms, especially those involving multiple electron shells, inner shell electrons act somewhat like a buffer. However, not all electrons shield equally effectively. When we discuss the lanthanides, which belong to the f-block of the periodic table, the 4f electrons provide particularly poor shielding.

• Inner shell electrons can block the nuclear charge, thereby affecting an outer electron's attraction to the nucleus.
• The less effective the inner electrons are at blocking the nuclear charge, the stronger the pull of the nucleus felt by the outer electrons.

In cases of poor shielding, such as amongst 4f electrons, outer electrons feel a stronger effective nuclear charge, which draws them closer to the nucleus and contributes to the lanthanide contraction.

4f Electrons

4f electrons occupy a subshell in the lanthanide series and possess unique characteristics that influence their shielding efficiency. This region of the electron configuration is notably different from s, p, or d electrons when it comes down to their behavior and interaction with nuclear attraction forces.

Electrons in the 4f subshell are poor at shielding, as they are more diffused and less able to block the nucleus' charge from reaching outer electrons.

• Since 4f orbitals lie deeper in the atom and are more internal, they are less effective in shielding outer electrons.
• The overlap of 4f orbitals with other electron orbitals is weak, leading to comparatively less effective blocking of nuclear charge.

This inefficiency in shielding is a pivotal factor in why the lanthanide ions decrease in size as you move from lanthanum (La) to lutetium (Lu) on the periodic table. The gradual increase in nuclear charge without effective shielding causes the outer electrons to be drawn closer, leading to the noticeable lanthanide contraction.

Effective Nuclear Charge

Effective nuclear charge refers to the net positive charge experienced by an electron in an atom. This is after accounting for the shielding effect from other electrons between the nucleus and the electron in question.

Lanthanides uniquely display the phenomenon of increasing nuclear charge with poor increase in shielding due to their filled 4f orbitals. Here's how:

• The actual nuclear charge increases with each subsequent element because protons are continually added to the nucleus as the atomic number increases.
• As the effective nuclear charge increases with poor 4f shielding, electrons in outer shells feel a stronger pull towards the nucleus.

This stronger effective nuclear charge in the case of lanthanides is largely responsible for the observable contraction in size across these elements as they do not expand widely outward, contributing to the decrease in ionic and atomic radii known as the lanthanide contraction.