Lanthanoid contraction is caused due to:

Options

A. The appreciable shielding on outer electrons by `4f` electrons from the nulcear charge
B. The appreciable shielding on outer electrons by `5f` electrons from the nulcear charge
C. the same effective nuclear charge from `Ce` to` Lu`
D. The imperfect shielding on outer electrons by `4f` electrons from the nulcear charge.

Explanation

To answer the question "Lanthanoid contraction is caused due to:", we can break down the explanation into several steps.

### Step-by-Step Solution:

1. **Understanding Lanthanoid Contraction**:
- Lanthanoid contraction refers to the gradual decrease in atomic and ionic radii of the lanthanide series elements as we move from cerium (Ce) to lutetium (Lu).

**Hint**: Remember that lanthanoid contraction is related to the change in size of the atoms as you move across the series.

2. **Nuclear Charge Increase**:
- As we move from Ce to Lu, the atomic number increases, which means the nuclear charge (the positive charge from protons in the nucleus) also increases.

**Hint**: Think about how an increase in nuclear charge affects the attraction between the nucleus and the electrons.

3. **Valence Electrons and Shielding Effect**:
- The valence electrons are pulled closer to the nucleus due to the increased nuclear charge. However, the 4f electrons, which are being filled in this series, provide poor shielding for the outer electrons.

**Hint**: Consider how effective shielding can influence the size of the atom.

4. **Poor Shielding by 4f Electrons**:
- The 4f subshell is diffused and does not effectively shield the outer electrons from the increased nuclear charge. This results in a stronger attraction between the nucleus and the outer electrons, leading to a greater decrease in size than expected.

**Hint**: Reflect on the characteristics of the f-orbitals and how they differ from s and p orbitals in terms of shielding.

5. **Conclusion**:
- The lanthanoid contraction is primarily caused by the imperfect shielding of outer electrons by the 4f electrons from the nuclear charge. This leads to a greater than expected reduction in atomic and ionic radii.

**Hint**: Summarize the relationship between shielding, nuclear charge, and atomic size to solidify your understanding.

### Final Answer:
Lanthanoid contraction is caused due to the imperfect shielding on outer electrons by 4f electrons from the nuclear charge.