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 4f 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: Understand the Lanthanoid Series

Lanthanoids, also known as lanthanides, are a series of 15 chemical elements from Lanthanum (La) to Lutetium (Lu) in the periodic table. They are known for having similar properties and primarily exhibit the +3 oxidation state.

Step 2: Analyze Option (a)

Statement (a) claims that all lanthanoids can form compounds in the +4 oxidation state. While some lanthanoids like Ce, Pr, and Tb can exhibit a +4 state, most lanthanoids predominantly show a +3 state and not consistently a +4 state.

Step 3: Analyze Option (b)

Statement (b) talks about the gradual decrease in the radii of lanthanoids with increasing atomic number, which is known as lanthanoid contraction. This is true because additional electrons are added to inner 4f orbitals which do not increase the shielding effect significantly.

Step 4: Analyze Option (c)

Statement (c) claims that separating lanthanoids is not easy due to their similar properties. This is accurate because the chemical and physical properties of lanthanoids are remarkably similar, making their separation challenging.

Step 5: Analyze Option (d)

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

Step 6: Identify the Incorrect Statement

Based on the analysis, statement (a) is incorrect because though some lanthanoids can form +4 oxidation states, it is not a general trait for all members of the series.

Key Concepts

Lanthanoid contraction

Lanthanoid contraction refers to the gradual decrease in size of the lanthanoid elements as their atomic number increases. As you move from Lanthanum (La) to Lutetium (Lu) in the periodic table:

• The atomic and ionic radii decrease steadily.
• This shrinkage is primarily because electrons are being added to the 4f orbital.
• The 4f electrons do not shield one another effectively from the increased positive charge of the nucleus.

This contraction is why, despite the addition of more protons and electrons, the overall size diminishes.

Lanthanoid contraction impacts both the physical and chemical properties of the elements that follow in the periodic table, especially the transition metals. This phenomenon also highlights differences in acidity and magnetic properties among lanthanoids.

Oxidation states

Oxidation states in chemistry indicate the degree of oxidation of an atom within a compound. For lanthanoids:

• The most stable and common oxidation state is +3, seen in nearly all lanthanoids.
• This state arises due to the loss of three electrons, primarily from the 6s and 5d orbitals.
• Certain lanthanoids, such as Cerium (Ce), Praseodymium (Pr), and Terbium (Tb), can reach a +4 oxidation state under specific conditions.

While the +3 state dominates, the presence of other oxidation states shows the versatility of these elements in forming various chemical compounds. It is noteworthy that the ability to exhibit +4 oxidation mainly occurs because the energy needed to remove an additional electron is available under certain circumstances.

Lanthanide series

The Lanthanide series is a set of 15 elements on the periodic table, ranging from Lanthanum (La) to Lutetium (Lu). These elements are characterized by having:

• Similar chemical properties due to their electron configurations.
• Primarily occupied 4f orbitals, although they are typically not involved in bonding.
• A common +3 oxidation state, which is stable and prevalent across the series.

Due to the similar sizes of their ions and nearly identical properties, lanthanides are notoriously challenging to separate from each other. They are vital in various applications, including electronics, optics, and as catalysts. Understanding the nuances of the Lanthanide series helps in appreciating their role in technology and industry, as well as their unique chemistry.