Problem 145 - A Complete Resource Book in Chemistry for JEE Main

Question

Larger number of oxidation states are exhibited by the actinoids than those by the lanthanoids, the main reason being (a) 4 f-orbitals more diffused than the 5 f-orbitals (b) Lesser energy difference between \(5 \mathrm{f}\) and 6 d than between \(4 \mathrm{f}\) and \(5 \mathrm{~d}\) orbitals (c) More energy difference between \(5 \mathrm{f}\) and \(6 \mathrm{~d}\) than between \(4 \mathrm{f}\) and \(5 \mathrm{~d}\) orbitals (d) More reactive nature of the actinoids than the lanthanoids.

Step-by-Step Solution

Answer

Option (b): Lesser energy difference between 5f and 6d than between 4f and 5d orbitals.

Step 1: Understand the Context

We need to compare the oxidation states of actinoids and lanthanoids, and determine why actinoids exhibit more oxidation states. The exercise lists different potential reasons for this behavior.

Step 2: Identify Key Factors

Focus on the characteristic features of actinoids and lanthanoids: the presence of 5f and 6d orbitals for actinoids and 4f and 5d orbitals for lanthanoids. The arrangement and energy levels of these orbitals influence their oxidation states.

Step 3: Evaluate Energy Differences

Consider the energy difference between the orbitals: For actinoids, the energy difference between 5f and 6d orbitals is smaller than the energy difference between 4f and 5d orbitals of lanthanoids. Smaller energy differences lead to a diverse range of electron transitions and oxidation states.

Step 4: Reason Out the Options

Option (b) suggests that there is a lesser energy difference between 5f and 6d orbitals than between 4f and 5d orbitals. This implies that more electrons can participate in bonding in actinoids, leading to diverse oxidation states. This matches our evaluation in the previous step.

Step 5: Select the Most Relevant Option

Based on the evaluation, select option (b) because it correctly explains why the actinoids have a greater range of oxidation states compared to lanthanoids.

Key Concepts

Oxidation States

Oxidation states describe the degree of oxidation of an atom in a compound. They reflect the number of electrons an atom can gain, lose, or share during chemical reactions.

• Actinoids, being similar to lanthanoids, belong to the f-block elements on the periodic table.
• However, they exhibit a wider range of oxidation states than lanthanoids.

This variance mainly stems from the unique properties of their 5f orbitals, which are more diffused and participate in bonding more extensively than the 4f orbitals of lanthanoids. As a result, actinoids can engage in varied oxidation states, facilitating complex chemical behavior and reactions.

f-orbitals

The f-orbitals are a set of orbitals found in elements of the f-block, such as lanthanoids and actinoids. These orbitals are characterized by their shape and size, impacting chemical properties.

• f-orbitals can hold up to 14 electrons, contributing to the complexity of these elements.
• In actinoids, the 5f orbitals are less shielded and more diffused compared to the 4f orbitals in lanthanoids.

This increased diffusion allows for greater electron interactions and bonding possibilities, explaining the expanded range of oxidation states for actinoids. Therefore, understanding f-orbitals is crucial for comprehending the distinct behaviors of these unique elements.

Electron Transitions

Electron transitions are movements of electrons between different energy levels or orbitals within an atom. These transitions play an essential role in explaining the rich chemistry of actinoids.

• Actinoids' 5f electrons can transition between the 5f, 6d, and other orbitals.
• This flexibility leads to more possible oxidation states and diverse chemical properties.

The ease of these transitions influences how readily electrons participate in bonding, contributing to the actinoids' varied oxidation states and reactivity.

Lanthanoids

Lanthanoids consist of 15 chemical elements from lanthanum to lutetium, known for their relatively uniform +3 oxidation state.

• Their electron configuration primarily involves 4f orbitals, which are more compact and less available for bonding compared to 5f orbitals.
• Due to similar outer electron arrangements, lanthanoids display less variation in oxidation states.

Despite their lesser diversity in chemistry compared to actinoids, lanthanoids are still significant, particularly for their magnetic and optical properties. Understanding these differences helps in comparing them with actinoids and explaining the range of compounds they form.

Energy Difference Between Orbitals

The energy difference between orbitals is critical in determining an element's chemical behavior and oxidation state variety. For actinoids and lanthanoids, these energy differences are pivotal in their chemistry.

• For actinoids, the energy difference between 5f and 6d orbitals is smaller, enabling more electronic transitions.
• In lanthanoids, larger energy differences between 4f and 5d orbitals restrict electron transitions.

This smaller energy gap in actinoids allows for greater flexibility in electron arrangements and subsequently broader oxidation states, reinforcing their complexity relative to lanthanoids. Understanding these differences aids in grasping the unique characteristics and reactivity of these fascinating elements.