Which of the following statements is not correct?

Options

A. `La(OH)_(3)` is less basic than `Li(OH)_(3)`
B. In lanthanide series, ionic radius of `Ln^(3+)` ion decreases
C. La is actually an element of transition series rather lanthanide
D. Atomic radius of Zr and Hf are same because of lanthanide contraction.

Explanation

To determine which statement is not correct, we will analyze each statement one by one.

### Step 1: Analyze the first statement
**Statement 1:** La(OH)₃ is less basic than Lu(OH)₃.

- **Explanation:** In the lanthanide series, as we move from La to Lu, the ionic size decreases due to the lanthanide contraction. A smaller ionic size means that the metal ion can hold onto hydroxide ions more effectively. Therefore, La(OH)₃, with a larger ionic size, will be more basic than Lu(OH)₃, which has a smaller ionic size. Thus, this statement is incorrect.

### Step 2: Analyze the second statement
**Statement 2:** In the lanthanide series, the ionic radius of Ln³⁺ decreases.

- **Explanation:** This statement is correct. The ionic radius of Ln³⁺ ions decreases across the lanthanide series due to the lanthanide contraction, which results from the poor shielding effect of the f-orbitals.

### Step 3: Analyze the third statement
**Statement 3:** Lanthanides are usually considered as elements of the transition series rather than lanthanides.

- **Explanation:** This statement is somewhat misleading. While lanthanides do have some transition metal characteristics, they are primarily classified as f-block elements. The last electron enters the f-subshell, not the d-subshell, which distinguishes them from transition metals. However, the statement can be interpreted in the context of their properties, so it is not entirely incorrect.

### Step 4: Analyze the fourth statement
**Statement 4:** The atomic radius of zirconium and hafnium is the same because of lanthanide contraction.

- **Explanation:** This statement is correct. The atomic radii of zirconium (Zr) and hafnium (Hf) are similar due to the lanthanide contraction, which affects the 5d and 6d series, leading to similar sizes.

### Conclusion
After analyzing all statements, the first statement is the one that is not correct.

**Final Answer:** The statement that is not correct is: **La(OH)₃ is less basic than Lu(OH)₃.**

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