Actinoid contraction is more than lanthanoid contraction because

Options

A. Energy of 4 f and 5f orbitals are same
B. 5 f orbitals are more diffused as compared to 4 f orbitals
C. Actinoids show greater range of oxidation states than lanthanoids
D. Actinoids are highly reactive

Answer

To understand why actinoid contraction is more than lanthanoid contraction, we need to analyze the concepts of shielding and contraction in both series.

### Step-by-Step Solution:

1. **Definition of Contraction**:
- Contraction refers to the decrease in atomic or ionic size as we move across a series due to the increasing nuclear charge which pulls the electrons closer to the nucleus.

2. **Understanding Lanthanoid Contraction**:
- The lanthanoid series consists of elements from lanthanum (La) to lutetium (Lu). In this series, the 4f orbitals are being filled.
- The 4f orbitals have poor shielding capabilities because they are relatively compact and do not effectively shield the nuclear charge from the outer electrons. This leads to a significant contraction as the effective nuclear charge increases.

3. **Understanding Actinoid Contraction**:
- The actinoid series consists of elements from actinium (Ac) to lawrencium (Lr). Here, the 5f orbitals are being filled.
- Similar to the lanthanoids, the 5f orbitals also have weak shielding. However, they are more diffused compared to the 4f orbitals, which means they are less effective in shielding the nuclear charge.

4. **Comparison of Shielding**:
- The shielding effect is quantified by the screening constant (σ). In the case of lanthanides, the 4f orbitals provide a certain level of shielding, while in actinides, the 5f orbitals provide even less shielding due to their diffused nature.
- Since the 5f orbitals are more diffused, they experience a greater effective nuclear charge, leading to a stronger contraction.

5. **Conclusion**:
- Therefore, the actinoid contraction is more pronounced than the lanthanoid contraction due to the weaker shielding effect of the 5f orbitals compared to the 4f orbitals. This results in a greater effective nuclear charge acting on the outer electrons in the actinoid series.

### Final Answer:
Actinoid contraction is more than lanthanoid contraction because the 5f orbitals are more diffused and provide weaker shielding compared to the 4f orbitals, leading to a greater effective nuclear charge and thus a more significant contraction.

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