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? (a) poor shielding of one of 4 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

(a) poor shielding of one of 4f electrons by another in the subshell.

Step 1: Understanding Lanthanide Contraction

Lanthanide contraction refers to the progressive decrease in the ionic radii and atomic radii of the lanthanide series elements (from Lanthanum to Lutetium), although they have an increasing atomic number.

Step 2: Analyzing Shielding in f-subshell

Shielding effect occurs when inner electrons block the attractive force of the nucleus from the outer electrons. In the case of lanthanides, we focus on the 4f subshell.

Step 3: Identifying Poor Shielding

The 4f electrons have a poor shielding effect due to their diffused orbitals. Consequently, they don't effectively shield each other from the nuclear charge.

Step 4: Evaluating Given Options

Look at the options given: (a) refers to poor shielding by 4f electrons. (b) refers to effective shielding (which is incorrect for 4f). (c) and (d) involve 5d electrons, which are not central to the main cause of lanthanide contraction.

Step 5: Drawing the Conclusion

The main cause of the lanthanide contraction is the poor shielding of the nuclear charge by the 4f electrons, leading to a decrease in size of the lanthanide atoms.

Key Concepts

Shielding Effect

The shielding effect is a fundamental concept in chemistry that explains how inner electrons can partially block the positive pull of the nucleus on outer electrons. In multi-electron atoms, inner electrons repel outer electrons, decreasing the effective nuclear charge felt by the outer electrons. - A strong shielding effect means that the outer electrons feel less attraction to the nucleus. - A poor shielding effect, on the other hand, results in a stronger nuclear attraction experienced by the outer electrons. In the lanthanide series, the 4f electrons are responsible for the shielding effect on the outer electrons. However, due to their diffused nature and complex shapes, the 4f electrons offer poor shielding from the nuclear charge, making the shielding effect crucial for understanding the lanthanide contraction.

Atomic Radii

Atomic radii refer to the size of an atom. It is typically measured from the nucleus to the outermost stable electron boundary. In the lanthanide series, there's a unique phenomenon known as the 'lanthanide contraction.' - As we move across the lanthanide series (from Lanthanum to Lutetium), the atomic radii decrease, despite an increase in atomic number. - This occurs due to the poor shielding effect of 4f electrons, which allows the nuclear charge to pull the electrons closer. Understanding atomic radii is essential when discussing trends in the periodic table. Changes in radii affect the chemical properties and reactivity of elements, which is evident in how the lanthanide contraction affects the elements that follow them in the periodic table.

4f Subshell

The 4f subshell is part of the f-block in the periodic table, occupied by the lanthanide elements. These elements have electrons filling the 4f orbital, influencing their chemical and physical properties considerably. - The 4f orbitals are more diffused compared to s, p, or d orbitals. This is why they provide poor shielding between each other. - As electrons fill the 4f subshell, the poorly shielded nuclear charge leads to a greater pull on outer electrons. The properties of the 4f subshell, including its filling and electron interactions, are vital for explaining why the lanthanide elements exhibit gradual changes in properties, stretching over the series, leading to the phenomenon known as lanthanide contraction.

Nuclear Charge

Nuclear charge refers to the total charge of the nucleus, consisting of protons. This charge significantly influences all electrons present within an atom. - An increase in nuclear charge normally results in a tighter hold on electrons. - Poor shielding of nuclear charge by 4f electrons means that the increase in nuclear charge is less counteracted as more electrons are added across the lanthanide series. Since the 4f electrons poorly shield each other, the increase in nuclear charge causes the atomic and ionic radii to decrease. This is the basis of the lanthanide contraction, underlying the decrease in size across the series from Lanthanum to Lutetium, and having major implications on chemical behavior and bonding.