Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

General electronic configuration of lanthanides is (a) \((n-2) f^{1-14}(n-1) s^{2} p^{6} d^{0-1} n s^{2}\) (b) \((n-2) f^{0-14}(n-1) d^{0-1} n s^{2}\) (c) \((\mathrm{n}-2) \mathrm{f}^{0-14}(\mathrm{n}-1) \mathrm{d}^{10} \mathrm{~ns}^{2}\) (d) \((n-2) f^{0-1}(n-1) f^{1-14} n s^{2}\)

Step-by-Step Solution

Answer

The correct answer is (a) \\( (n-2) f^{1-14} (n-1) s^{2} p^{6} d^{0-1} n s^{2} \\).

Step 1: Understanding the Lanthanides

The lanthanides, also known as the rare earth elements, include the 15 elements with atomic numbers from 57 to 71, from lanthanum to lutetium. These elements belong to the f-block of the periodic table, where electrons are added to the 4f orbital.

Step 2: Identifying the Electron Configuration Pattern

The general electronic configuration for lanthanides is characterized by the filling of 4f orbitals. As you proceed across the period, electrons typically fill the \(n-2)f\) orbital, where \(n\) represents the principal quantum number that corresponds to the valence shell.

Step 3: Analyzing Given Options

We must identify the correct form of electron configuration from the options given. Note that for lanthanides: \(n-2)f^{1-14}\) aligns with the filling of 4f orbitals, \(n-1)s^{2}p^{6}d^{0-1}\) represents the filled 5s, 5p orbitals and partially filled 5d, and \(ns^{2}\) the filled 6s orbital for the counted valence electrons.

Step 4: Selecting the Correct Configuration

Based on the typical pattern for lanthanide electron configurations, option (a) \( (n-2) f^{1-14} (n-1) s^{2} p^{6} d^{0-1} n s^{2} \) fits the observed pattern, as it highlights the primary filling in the 4f block, accompanied by the complete 5s, 5p and initiating the 6s block.

Key Concepts

Electronic Configuration

Understanding the electronic configuration of elements, especially for the lanthanides, helps us predict their chemical properties and behaviors. The term "electronic configuration" refers to the distribution of electrons among the atomic orbitals of an atom. For lanthanides, this configuration has a specific pattern due to their position in the periodic table—specifically in the f-block.
The electronic configuration of lanthanides typically follows this pattern:

• Electrons first fill the 4f orbitals, which can hold up to 14 electrons, as indicated by terms like \( (n-2)f^{1-14} \).
• This is followed by filling the 5s and 5p orbitals (\( n-1)s^{2}p^{6} \)).
• Additionally, the 5d orbital of these elements can hold up to one electron (\(d^{0-1}\)), though the 4f block is more customary.

This organized approach in electron filling influences how these elements interact and react with other elements, giving them their unique "rare earth" properties.

F-block elements

The periodic table is divided into blocks based on electron filling sequences, and one of these blocks is the f-block, where the lanthanides reside. F-block elements are characterized by the filling of f orbitals. Amongst the periodic table divisions—which also consist of the s, p, and d blocks—the f-block represents a segment of elements where electrons fill the 4f and 5f subshells. This location in the periodic table is commonly known as the "inner transition metals."
This unique categorization arises because:

• The 4f subshell, where the f-block lanthanides fall, is being filled with electrons as we move across the series.
• These orbitals are shielded by the filled s and p orbitals, which affects both their chemical reactivity and physical properties.

Thus, the f-block elements, particularly lanthanides, show trends and behaviors distinct from the more familiar s, p, and d block elements, often manifesting in magnetic and spectroscopic properties that make them integral to modern technology applications.

4f orbitals

4f orbitals play a crucial role when studying lanthanides. These orbitals are parts of the atomic structure where electrons reside, and they are particularly significant because they lie deeper within the atom than s, p, and d orbitals.
Here's why 4f orbitals are important:

• They can hold up to 14 electrons because they correspond to the quantum number l=3, which allows for 7 different orientations of these orbitals.
• Electrons that are added to these orbitals typically determine the magnetic and spectral characteristics of the lanthanides.

4f orbitals are often shielded by the filled 5s and 5p orbitals, resulting in some unique chemical behaviors. This shielding influences how the electrons in 4f orbitals participate in bonding and how they affect the physical properties of lanthanides.

Rare earth elements

The term "rare earth elements" can be somewhat misleading as it suggests scarcity, though many of these elements are plentiful. The lanthanides, which are key representatives of rare earth elements, include the elements from lanthanum to lutetium in the periodic table.
Here are some characteristics:

• Despite their collective name, these elements are not exactly rare but are more challenging to extract due to their dispersed distribution.
• They have unique properties, including exceptional magnetic, luminescent, and electrochemical abilities.

Due to these attributes, rare earth elements are essential for numerous advanced technologies. They are crucial in producing powerful magnets, phosphors in lighting and displays, and as catalysts in various chemical reactions.