Problem 144 - A Complete Resource Book in Chemistry for JEE Main

Question

Lanthanoid contraction is caused due to (a) the appreciable shielding of outer electrons by \(4 \mathrm{f}\) electrons from the nuclear charge (b) the appreciable shielding on outer electrons by 5 d-electrons from the nuclear charge (c) the same effective nuclear charge from \(\mathrm{Ce}\) to \(\mathrm{Lu}\) (d) the imperfect shielding of outer electrons by \(4 \mathrm{f}\) electrons from the nuclear charge

Step-by-Step Solution

Answer

Lanthanoid contraction is caused by imperfect shielding by 4f electrons.

Step 1: Understanding Lanthanoid Contraction

Lanthanoid contraction refers to the gradual decrease in the size of the lanthanoid ions as the atomic number increases. This contraction is observed across the lanthanide series from cerium (Ce) to lutetium (Lu). To understand the cause, we need to pinpoint how the electrons are interacting with the nuclear charge.

Step 2: Evaluate Option A

Option (a) suggests that the lanthanoid contraction is due to the appreciable shielding of outer electrons by 4f electrons from the nuclear charge. If 4f electrons provided strong shielding, the increase in nuclear charge would not significantly affect outer electron attraction.

Step 3: Evaluate Option B

Option (b) states the appreciable shielding of outer electrons by 5d electrons. However, 5d electrons provide significant shielding, which is unrelated to the lanthanoid contraction as it primarily concerns the 4f subshell and its inefficiency.

Step 4: Evaluate Option C

Option (c) posits a consistent effective nuclear charge from Ce to Lu. If the effective nuclear charge were constant, we would not observe significant changes like size reduction along the series.

Step 5: Evaluate Option D

Option (d) describes the imperfect shielding of outer electrons by 4f electrons from the nuclear charge. This is accurate because 4f electrons are not efficient at shielding, meaning the increased nuclear charge pulls the outer electrons closer, resulting in size reduction.

Step 6: Choose the Best Explanation

Among the options, option (d) is the most accurate, as it correctly attributes lanthanoid contraction to the imperfect shielding by 4f electrons. This allows the increased nuclear charge to effectively reduce the size of the atom across the series.

Key Concepts

Shielding Effect

The shielding effect refers to the ability of inner electrons to reduce the effective nuclear charge experienced by outer electrons. When electrons reside between an outer electron and the nucleus, they repel the outer electron, decreasing the force of attraction from the nucleus. This phenomenon plays a critical role in determining atomic sizes across periods in the periodic table.
In the case of lanthanoids, the 4f electrons are responsible for shielding. However, they are not highly effective at it. This inefficacy stems from their position deep within the electron cloud, which does not completely block the nuclear charge from acting on the valence electrons. The result is that outer electrons remain more tightly bound to the nucleus, compared to more efficient shielding electrons like those in s or d orbitals.

4f Electron Configuration

The 4f electron configuration is a key player in the behavior of lanthanoid elements. As atomic number increases from cerium (Ce) to lutetium (Lu), each element in the lanthanide series adds an extra electron to the 4f subshell. This results in progressive changes in both electronic structure and physical properties.
The 4f subshell is nestled further inward compared to outer shells, leading to a reduced impact in screening the nuclear charge. Due to their poor shielding capability, the influence of protons on outer electrons intensifies, exerting a greater pull and thus contributing to the lanthanoid contraction. Understanding the 4f configuration is essential to fully grasp interactions within and beyond the lanthanoids.

Atomic Radii Trend

The atomic radii trend seen in the lanthanide series is peculiar compared to other series in the periodic table. Generally, as we move across a period, atomic radii decrease due to an increasing nuclear charge pulling electrons closer. However, in the lanthanide series, a noticeable contraction occurs known as lanthanoid contraction.
This phenomenon is primarily due to the ineffective shielding by 4f electrons. Despite adding electrons to the 4f subshell across the series, the rising nuclear charge is not sufficiently counterbalanced. Thus, the increasing attraction results in a continual decrease in the size of the atoms. This trend has significant implications for the chemical properties and coordination chemistry of the lanthanoids.

Nuclear Charge

Nuclear charge is the total positive charge within a nucleus, equal to the number of protons present. As you progress through the periodic table, nuclear charge rises with each added proton. In the lanthanide series, the nuclear charge has notable implications due to interactions with both inner 4f and outer electrons.
The essence of lanthanoid contraction stems from the rising nuclear charge not being appropriately countered by the 4f electron shielding. This leads to more pronounced attractions between the nucleus and outer electrons, effectively reducing the atomic and ionic size across the lanthanoids. The concept of nuclear charge is critical when examining the compressive effect observed in this series.

Lanthanide Series

The lanthanide series comprises 15 metallic elements, spanning atomic numbers 57 through 71, from lanthanum (La) to lutetium (Lu). Known for their silver-like appearance and high melting points, they exhibit a striking array of chemical and physical properties. The term lanthanides is somewhat misleading, as it implies a lighter classification; nonetheless, these elements have electrons filling the 4f orbitals.
These elements collectively showcase unique trends, including the lanthanoid contraction, which links back to their shared 4f configuration. Beyond academic intrigue, understanding this series has practical utility. Lanthanides are pivotal in industries, contributing to the creation of strong magnets, catalysts, phosphors in LED and fluorescent lighting, and even in advanced technologies like hybrid vehicles.