Lanthanoid contraction is due to increase in

Options

A. atomic number
B. effective nuclear charge
C. atomic radius
D. valence electrons.

Answer

### Step-by-Step Solution:

1. **Understanding Lanthanoid Contraction**:
- Lanthanoid contraction refers to the gradual decrease in the size of the lanthanide ions as we move from lanthanum (La) to lutetium (Lu) in the periodic table.

2. **Identifying the Cause of Contraction**:
- The contraction occurs due to an increase in the effective nuclear charge (Z_eff) experienced by the electrons in the outermost shell.

3. **Effective Nuclear Charge (Z_eff)**:
- The effective nuclear charge is the net positive charge experienced by an electron in a multi-electron atom. It takes into account the actual nuclear charge (total number of protons) and the shielding effect caused by inner-shell electrons.

4. **Impact of Z_eff on Atomic Size**:
- As the effective nuclear charge increases, the attraction between the nucleus and the outer electrons becomes stronger. This results in the electrons being pulled closer to the nucleus, leading to a decrease in atomic and ionic size.

5. **Comparison with Other Options**:
- **Atomic Number**: While the atomic number increases as we move across the lanthanides, it is not the direct reason for the contraction.
- **Atomic Radius**: The atomic radius decreases as a result of the increase in effective nuclear charge, not the cause of it.
- **Valence Electrons**: The number of valence electrons does not directly cause the contraction; it is the effective nuclear charge that influences the size.

6. **Conclusion**:
- Therefore, the correct answer to the question is that lanthanoid contraction is due to an increase in the effective nuclear charge.

### Final Answer:
**Effective Nuclear Charge (Option B)**

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