In context of the lanthanoids , which of the following statement is not correct?

Options

A. There is a gradual decrease in the radii of the members with increasing atomic number in the series
B. All the member exhibit `+3` oxidation state
C. Availability of `4f` electrons result in the formation of compounds in `+4` state for all member of the series
D. Because of the similar property, separation of lanthanoids is not easy.

Explanation

To solve the question regarding the lanthanoids and identify which statement is not correct, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: There is a gradual decrease in the radii of the members with the increased atomic number in the series.**
- **Analysis:** As we move from cerium (Ce) to lutetium (Lu) in the lanthanoid series, the atomic and ionic radii do indeed show a gradual decrease. This is due to the increased nuclear charge (more protons) pulling the electrons closer to the nucleus, and the poor shielding effect of the 4f electrons.
- **Conclusion:** This statement is **correct**.

2. **Statement 2: All the members exhibit plus 3 oxidation state.**
- **Analysis:** The most common oxidation state for lanthanides is +3. While some lanthanides can exhibit +2 and +4 oxidation states, +3 is predominant due to the relatively low ionization energy required to remove three electrons.
- **Conclusion:** This statement is **correct**.

3. **Statement 3: Availability of 4f electrons results in the formation of compounds in plus 4 states for all members of the series.**
- **Analysis:** Not all lanthanides can achieve a +4 oxidation state. The formation of a +4 state requires a significant amount of energy to remove four electrons, which is not feasible for all lanthanides. Therefore, this statement is incorrect.
- **Conclusion:** This statement is **not correct**.

4. **Statement 4: Because of the similar property, separation of lanthanoids is not easy.**
- **Analysis:** Lanthanides have similar chemical properties due to their similar size and electronic configuration, making their separation through conventional methods difficult. This statement is true.
- **Conclusion:** This statement is **correct**.

### Final Answer:
The statement that is **not correct** is **Statement 3**: "Availability of 4f electrons results in the formation of compounds in plus 4 states for all members of the series."

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