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 \([2008]\) (a) 4 f-orbitals more diffused than the 5 f-orbitals (b) Lesser energy difference between \(5 \mathrm{f}\) and \(6 \mathrm{~d}\) 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: Understanding the Oxidation State

The oxidation state is an indicator of the degree of oxidation or reduction of an element in a compound. Transition metals, including lanthanoids and actinoids, show variable oxidation states because of the involvement of their d and f orbitals in bonding. Here we compare lanthanoids (involving 4f and 5d orbitals) and actinoids (involving 5f and 6d orbitals).

Step 2: Electronic Configuration Insight

Lanthanoids involve the filling of 4f orbitals and actinoids involve the filling of 5f orbitals. The electrons in these orbitals participate in bonding to different extents. Generally, f-orbitals are less shielded and more diffused than d-orbitals, making them more accessible for bonding and influencing oxidation states.

Step 3: Energy Difference Analysis

For actinoids, the energy difference between 5f and 6d orbitals is lesser than between 4f and 5d orbitals in lanthanoids. This smaller energy gap in actinoids allows for easier promotion of electrons from 5f to 6d and vice versa, leading to a wider variety of oxidation states. Thus, option (b) is a potential reason for greater oxidation states in actinoids.

Step 4: Evaluating Given Options

Option (a) suggests 4f orbitals are more diffused than 5f, but it's actually the opposite; 5f orbitals are more diffuse, allowing variety in bonding. Option (c) contradicts known properties, as lesser energy difference, not more, facilitates greater oxidation states. Option (d) is unrelated to the oxidation state variety.

Step 5: Final Conclusion

Option (b) correctly suggests that the smaller energy difference between 5f and 6d orbitals compared to 4f and 5d is the main reason actinoids exhibit a larger number of oxidation states compared to lanthanoids.

Key Concepts

Oxidation States in Actinides

Actinides exhibit a variety of oxidation states, which are the charges an element can have in a compound. This is largely due to the unique configuration of their electrons. The f-orbitals in actinides play a crucial role in exhibiting higher oxidation states. These states can range widely, allowing actinides to form diverse compounds. What makes actinides different from lanthanoids is their tendency to show more such states because their f-orbitals are more readily available for bonding. Understanding oxidation states helps to predict how these elements will react chemically. This is not just useful for academic exercises but also for real-world applications in nuclear chemistry and materials science.

• Actinides: Variable oxidation states due to f-orbitals.
• Lanthanoids: Fewer oxidation states in comparison.
• Practical applications: Useful in nuclear chemistry.
• Academic importance: Essential for understanding chemical behavior.

Electronic Configuration of Actinides

Electronic configuration explains how electrons are distributed in an atom's orbitals. For actinides, the filling of the 5f orbitals is key. The lanthanoids fill the 4f orbitals, leading to differences in chemical properties. In actinides, as the 5f orbitals are progressively filled, they become available for overlapping with other orbitals, enhancing bonding capabilities and oxidation potential. The electrons in both 5f and 6d orbitals are involved, unlike in lanthanoids where mostly 4f orbitals participate. This extended participation of 5f and 6d orbitals in bond formation is central to the actinides' chemistry.

Actinides vs. Lanthanoids

• Actinides: Filling of 5f orbitals.
• Lanthanoids: Filling of 4f orbitals.
• Bonding: 5f orbitals allow more complex bonding.
• Chemical properties: Enhanced due to electronic configuration.

Energy Difference Analysis

The energy differences between orbitals significantly impact the chemical behavior of elements. In actinides, there is a smaller energy difference between the 5f and 6d orbitals compared to the lanthanoids, where the difference is larger between the 4f and 5d orbitals. This smaller energy gap in actinides facilitates the transition of electrons, enhancing the variety and stability of oxidation states. This ease of electron transition is a reason why actinides can exhibit multiple oxidation states. This concept is crucial for predicting reactivity and stability in chemical reactions and processes.

Key Points

• Actinides: Smaller energy difference between 5f and 6d.
• Lanthanoids: Larger energy difference between 4f and 5d.
• Impact: Easier electron transition in actinides.
• Result: Greater variety in oxidation states.

The Role of f-Orbitals

f-Orbitals are a type of orbital that is more complex and less understood than the standard s, p, and d orbitals. In the context of actinide chemistry, the 5f orbitals are significant because they are more diffused and less shielded compared to 4f orbitals. This means they can participate more actively in chemical bonding. This active participation results in actinides displaying more oxidation states. The f-orbitals are often involved in complex chemical behavior due to their ability to form diverse bonds with various elements, which is a unique property aiding in the understanding of these heavy elements.

Characteristics of f-Orbitals

• Complex structure and behavior.
• Actinides: 5f orbitals are more active in bonding.
• Diffusion: 5f orbitals are more diffused than 4f.
• Influences: Help in exhibiting diverse oxidation states.