Problem 149 - A Complete Resource Book in Chemistry for JEE Main

Question

In context of the lanthanoids, which of the follwoing statements is not correct? (a) Availability of \(4 \mathrm{f}\) electrons results in the formation of compound in \(+4\) state for all the members of the series. (b) There is a gradual decrease in the radii of the members with increasing atomic number in the series. (c) Because of similar properties the separation of lanthanoids is not easy (d) All the members exhibit \(+3\) oxidation state.

Step-by-Step Solution

Answer

The incorrect statement is (a).

Step 1: Analyze Statement (a)

Statement (a) claims that the availability of 4f electrons results in the formation of a +4 oxidation state for all lanthanoids. While some lanthanoids can exhibit a +4 oxidation state, not all of them do. Therefore, this statement is not generally correct.

Step 2: Evaluate Statement (b)

Statement (b) indicates a gradual decrease in the radii of lanthanoids with increasing atomic number, known as the lanthanide contraction. This statement is true and is an established trend across the series.

Step 3: Check Statement (c)

Statement (c) suggests that due to their similar properties, separating lanthanoids is challenging. This is true because the chemical and physical similarities make separation difficult.

Step 4: Verify Statement (d)

Statement (d) asserts that all lanthanoids exhibit a +3 oxidation state. This is correct, as +3 is the most stable and common oxidation state for lanthanoids.

Step 5: Identify Incorrect Statement

Among the provided options, statement (a) is not correct as it incorrectly claims that all lanthanoids form compounds in the +4 oxidation state.

Key Concepts

4f electrons

The lanthanoids, also known as lanthanides, include elements with atomic numbers 57 through 71. A characteristic feature of these elements is the presence of partially filled 4f orbitals. These 4f electrons are buried deep within the atom, beneath the outer 5d and 6s orbitals.
Their participation in bonding is limited due to their core-like nature. However, they significantly influence the chemical properties of lanthanoids.

• The shielding effect of 4f electrons is poor. This leads to a greater effective nuclear charge experienced by the outer electrons.
• The unique electron configuration involving 4f electrons contributes to the lanthanoids being f-block elements.

Despite their limited direct involvement in chemical bonding, the 4f electrons result in complex behaviors of lanthanoids, affecting their magnetic and optical properties.

Lanthanide Contraction

Lanthanide contraction refers to the progressive decrease in ionic radii and atomic sizes of lanthanoids as the atomic number increases from lanthanum to lutetium.
This trend occurs despite the increasing positive charge of the nucleus. The main reason is the inefficient shielding of nuclear charge by the 4f electrons.
Some important points about lanthanide contraction are:

• The ineffective shielding by 4f electrons does not fully compensate for the increasing nuclear charge.
• This makes the outer electrons more strongly attracted, reducing the size of the atoms and ions progressively.
• Lanthanide contraction significantly affects the chemistry of heavier main group elements and transition metals.

The contraction influences the chemical similarity among the lanthanoids, accounting for their nearly indistinguishable chemical properties.

Oxidation States

Lanthanoids predominantly exhibit a +3 oxidation state across the series.
This is a common feature resulting from the loss of two 6s electrons and one 4f electron during ionization. However, some lanthanoids can also exist in other oxidation states, most notably +2 and +4.
Key details about lanthanoids' oxidation states include:

• The +3 state is highly stable, stemming from a full or half-filled subshell stability concept.
• Only a few lanthanoids, like cerium, exhibit a stable +4 state under specific conditions.
• The +2 oxidation state appears in a few lanthanoids, such as europium and ytterbium, contributing to their unique chemistry.

Understanding these oxidation states is essential for grasping how lanthanoids interact with other elements and comprise various chemical compounds.

Separation of Lanthanoids

Separating lanthanoids is notoriously challenging due to their nearly identical chemical and physical properties.
They share comparable ionic radii and mostly exhibit the same oxidation state, resulting in overlapping chemical behavior. This makes conventional separation techniques ineffective.
Methods and challenges in separating lanthanoids are:

• Separation requires advanced techniques like ion-exchange chromatography, solvent extraction, and fractional crystallization.
• Even small differences in their ionic sizes and tendencies for complexation must be exploited for separation.
• These separation methods are intricate and require careful handling due to the subtlety of lanthanoid chemistry.

The intricate process results in the high cost of lanthanoids, despite their utility in various technological applications.