Lanthanoid contraction is caused due to

Options

A. Atomic number
B. Size of 4f orbitals
C. Effective nuclear charge
D. Poor shielding effect of 4f electrons

Explanation

**Step-by-Step Solution:**

1. **Understanding Lanthanoid Contraction**:
- Lanthanoid contraction refers to the gradual decrease in the ionic and atomic sizes of the lanthanide series elements from lanthanum (La) to lutetium (Lu).

2. **Role of 4f Orbitals**:
- The lanthanides involve the filling of the 4f orbitals. As we move from lanthanum to lutetium, the 4f orbitals are progressively filled.

3. **Poor Shielding Effect**:
- The 4f electrons have a poor shielding effect on the outermost electrons. This means that the outer electrons do not effectively shield the nuclear charge from the nucleus.

4. **Increased Effective Nuclear Charge**:
- Due to the poor shielding by the 4f electrons, the outermost electrons experience a stronger effective nuclear charge. As a result, the attraction between the nucleus and the outer electrons increases.

5. **Resulting Size Contraction**:
- This increased attraction causes the outermost electrons to be pulled closer to the nucleus, leading to a decrease in atomic and ionic radii, which is observed as lanthanoid contraction.

6. **Conclusion**:
- Therefore, the lanthanoid contraction is primarily caused by the poor shielding effect of the 4f electrons, leading to an increased effective nuclear charge experienced by the outer electrons.

**Final Answer**: Lanthanoid contraction is caused due to the poor shielding effect of 4f electrons.

---