Lanthanide contraction is caused due to -

Options

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

Answer

To answer the question about the cause of lanthanide contraction, we can follow these steps:

### Step 1: Understand Lanthanide Contraction
Lanthanide contraction refers to the phenomenon where the ionic radii of the lanthanide elements decrease as the atomic number increases, despite the addition of electrons to the 4f subshell.

### Step 2: Identify the Role of 4f Electrons
The 4f electrons are added as we move across the lanthanide series. These electrons have a poor shielding effect on the outer electrons due to their diffused shape.

### Step 3: Explain Poor Shielding
The poor shielding occurs because the 4f orbitals are more diffused compared to the s and p orbitals. This means that they do not effectively shield the outer electrons from the increasing nuclear charge as we move across the series.

### Step 4: Analyze the Options
Now, let's analyze the options provided in the question:
1. **Imperfect shielding of outer electrons by 4f electrons from the nuclear charge** - This is correct as it describes the poor shielding effect.
2. **Appreciable shielding of outer electrons** - This is incorrect; the shielding is not appreciable.
3. **Appreciable shielding of 5d electrons** - This is also incorrect; the shielding effect is not appreciable.
4. **Same effect in nuclear charge** - This option does not accurately describe the phenomenon.

### Step 5: Conclusion
Based on the analysis, the correct answer is that lanthanide contraction is caused by the imperfect shielding of outer electrons by 4f electrons from the nuclear charge.

### Final Answer
**The correct answer is: Imperfect shielding of outer electrons by 4f electrons from the nuclear charge.**

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