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? \(\quad\) (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 \(4 \mathrm{f}\) electrons

Step-by-Step Solution

Answer

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

Step 1: Understanding Lanthanide Contraction

Lanthanide contraction refers to the phenomenon where there is a gradual decrease in the radii of lanthanide elements as we move from left to right in the periodic table. This contraction is because of the poor shielding effect of the electrons in the 4f subshell.

Step 2: Assessing the Options

Consider the options given in the problem. We must determine which of these correctly explains the main cause of lanthanide contraction. This involves understanding the impact of shielding by 4f electrons.

Step 3: Reviewing Shielding Concepts

The effectiveness of electron shielding depends on the position and type of electrons. 4f electrons have a poor shielding effect because their probability distributions are not very effective at blocking the nuclear charge from reaching the outer electrons.

Step 4: Eliminating Incorrect Options

Options (b) and (d) involve effective or greater shielding, which are incorrect as 4f electrons do not effectively shield. Similarly, option (c) suggests 5d electron shielding is affected, which is secondary to the primary cause.

Step 5: Selecting the Correct Option

Since the poor shielding of 4f electrons leads to increased effective nuclear charge and thus, contraction in size, option (a) describes this correctly. The main cause of lanthanide contraction is indeed the poor shielding of 4f electrons.

Key Concepts

Electron Shielding

One of the important concepts to understand when studying elements and their behaviors is "electron shielding." This phenomenon occurs when inner electrons block or shield the outer electrons from experiencing the full positive charge of the nucleus.
It plays a significant role in determining the effective nuclear charge felt by outer electrons. The ability of inner electrons to shield outer electrons determines how tightly these outer electrons are held.
In lanthanide contraction, the 4f electrons have a poor shielding effect, meaning they are not very effective at reducing the nuclear pull on outer electrons. As a result:

• 4f electrons are less efficient at shielding compared to s or p electrons.
• The outer electrons feel more of the nuclear charge, causing the electrons to be pulled closer to the nucleus.

Understanding electron shielding helps to explain trends across the periodic table, such as the gradual decrease in ionic radii across the lanthanide series.

4f Subshell

In the context of the lanthanide elements, the 4f subshell is crucial. The penetration and electron distribution in this subshell significantly affect the properties of these elements.
The lanthanides fill the 4f orbitals, and electrons in this subshell exhibit specific behaviors due to their particular positioning. The 4f electrons are "buried" beneath s, p, and d orbitals, leading to their poor ability to shield other electrons effectively.
Because the 4f electrons do not shield well, the effective nuclear charge experienced by outer electrons is increased.
Key points about the 4f subshell:

• Electrons in the 4f subshell have complex wave functions that are not conducive to strong shielding.
• This results in a significant increase in effective nuclear charge, contributing to the lanthanide contraction.

The unique properties of the 4f subshell are essential in understanding why the lanthanide series exhibits the particular trends that it does.

Effective Nuclear Charge

"Effective nuclear charge" is a vital concept to grasp when discussing atomic size and shielding. This term refers to the net positive charge experienced by outer electrons, after accounting for any shielding provided by inner electrons.
The higher the effective nuclear charge, the more strongly the nucleus attracts the outer electrons, pulling them closer and reducing the atomic radius.
In the lanthanide series, due to the poor shielding by 4f electrons:

• The effective nuclear charge is relatively high for outer electrons.
• This increased nuclear attraction leads to a gradual decrease in atomic size as you move across the series.

Understanding how effective nuclear charge works allows for a clearer explanation of size variations and periodic trends, like the lanthanide contraction.

Periodic Table Trends

Analyzing trends within the periodic table, such as lanthanide contraction, requires a detailed understanding of how electron configurations impact atomic and ionic radii.
As you move from left to right across the lanthanide series, there's a noticeable decrease in ionic radii. This results from increasing effective nuclear charge due to poor shielding by 4f electrons. Key periodic trends affected by lanthanide contraction include:

• Atomic and ionic size decreases across a period.
• Chemical properties show subtle variations related to ionization energies and reactivity.

Understanding these periodic trends provides important insights into the behavior of elements, influencing their chemical and physical properties. Lanthanide contraction serves as a textbook example of how electron configuration and nuclear charge interplay affect the elemental characteristics observable in the periodic table.