Which of the following factor may be regarded as the main cause of lanthanide contraction?

Options

A. Poor shielding of `4` f-electrons in compare to other electrons in the sub-shell.
B. Effective shielding of one of the `4`f-electrons by another in the sub shell.
C. Poorer shielding of `5` d electrons by `4`f-electrons.
D. Greater shielding of `5f` d-electrons by `4` f-electrons.

Explanation

To determine the main cause of lanthanide contraction, we need to analyze the factors affecting the atomic size of lanthanides as we move across the series. Here’s the step-by-step solution:

### Step 1: Understand Lanthanide Contraction
- Lanthanide contraction refers to the gradual decrease in the size of lanthanide ions as we move from cerium (Ce) to lutetium (Lu) in the periodic table.

### Step 2: Analyze the Atomic Structure
- As we move across the lanthanide series, the atomic number increases. This means that the nuclear charge (the positive charge from protons in the nucleus) also increases.

### Step 3: Consider Electron Configuration
- The electrons being added as we move across the series are primarily filling the 4f subshell. Each element has one more electron in the 4f subshell than the previous element.

### Step 4: Evaluate Shielding Effect
- The 4f subshell is known to be diffused and poorly shields the nuclear charge. This means that the electrons in the 4f subshell do not effectively counteract the increasing nuclear charge from the protons.

### Step 5: Compare Shielding and Nuclear Charge
- Due to the poor shielding effect of the 4f electrons, the increase in nuclear charge is felt more strongly. Thus, the effective nuclear charge experienced by the outer electrons increases, leading to a greater attraction between the nucleus and the electrons.

### Step 6: Conclusion on Atomic Size
- As a result of the increased effective nuclear charge and the poor shielding by the 4f electrons, the atomic size of lanthanides decreases as we move from Ce to Lu.

### Step 7: Identify the Correct Option
- Given the options, the main cause of lanthanide contraction is the "poor shielding of 4f electrons compared to other electrons in the subshell." Therefore, this is the correct answer.

### Final Answer
The main cause of lanthanide contraction is the poor shielding of 4f electrons compared to other electrons in the subshell.

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