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

Options

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

Answer

To determine which statement about lanthanoids is not correct, we will analyze each of the provided statements step by step.

### Step 1: Analyze the First Statement
**Statement:** "The availability of 4f electrons results in the formation of compounds in the +4 state of all the members of the series."

**Analysis:**
- The characteristic oxidation state of lanthanoids is +3, not +4 for all members. While some lanthanoids can exhibit +4 oxidation states (like cerium and praseodymium), it is not true for all members of the series. Therefore, this statement is incorrect.

### Step 2: Analyze the Second Statement
**Statement:** "There is a gradual decrease in radii of the members with increasing atomic number in the series."

**Analysis:**
- This statement is true. The phenomenon known as "lanthanoid contraction" describes the gradual decrease in ionic radii as we move from lanthanum to lutetium. This occurs due to poor shielding by the f-electrons, which results in a stronger effective nuclear charge felt by the outer electrons.

### Step 3: Analyze the Third Statement
**Statement:** "All the members exhibit +3 oxidation state."

**Analysis:**
- This statement is also true. The +3 oxidation state is indeed the most stable and characteristic oxidation state for all lanthanoids without exception.

### Step 4: Analyze the Fourth Statement
**Statement:** "Because of similar properties, the separation of lanthanoids is not easy."

**Analysis:**
- This statement is correct. The lanthanoids have very similar chemical properties due to their comparable sizes and electronic configurations, making their separation quite challenging.

### Conclusion
After analyzing all four statements, we find that the first statement is the only incorrect one. Therefore, the answer to the question is:

**Answer:** The first statement is not correct.