Which of the following is not correct?

A. \(La(OH)_3\) is less basic than \(Lu(OH)_3\)

B. In lanthanoid series ionic radius of \(M^{3+}\) ion decreases from \(La\) to \(Lu\)

C. \(La\) is an element of transition series rather than a lanthanoid series.

D. Atomic radii of \(Zr\) and \(Hf\) are the same because of lanthanoid contraction

Choose the correct answer from the options given below:

A only

The correct answer is option 3. A only.

Let us go through each statement to explain why the correct answer is option 3: A only.

Statement A: \(La(OH)_3\) is less basic than \(Lu(OH)_3\)

The basicity of hydroxides in the lanthanoid series decreases from \(La(OH)_3\) to \(Lu(OH)_3\). This trend is due to the lanthanoid contraction, where the ionic radius of the lanthanoids decreases as we move from \(La\) to \(Lu\).

Lanthanoid Contraction: As the atomic number increases, the effective nuclear charge increases, pulling the electrons closer to the nucleus and reducing the ionic radius.

Effect on Basicity: The smaller the ionic radius, the less able the ion is to release hydroxide ions \((OH^-)\), making the hydroxide less basic. This means that \(La(OH)_3\) (with a larger ionic radius) is more basic than \(Lu(OH)_3\) (with a smaller ionic radius).

Why Statement A is Incorrect:

Incorrect Reasoning: The statement claims that \(La(OH)_3\) is less basic than \(Lu(OH)_3\), which is the opposite of the actual trend. In reality, \(La(OH)_3\) is more basic than \(Lu(OH)_3\).

Conclusion: Since the statement contradicts the actual trend, Statement A is incorrect.

Statement B: In the lanthanoid series, the ionic radius of \(M^{3+}\) ions decreases from \(La\) to \(Lu\)

Lanthanoid Contraction: As you move across the lanthanoid series from lanthanum (\(La\)) to lutetium (\(Lu\)), the ionic radius of the \(M^{3+}\) ions gradually decreases. This phenomenon is due to the increasing nuclear charge, which pulls the electrons closer to the nucleus despite the electrons being added to the 4f orbital.

Importance: This contraction is significant because it affects the chemical properties of the lanthanoids, as well as the elements that come after them in the periodic table.

Why Statement B is Correct:

Correct Trend: The statement accurately describes the observed trend in the lanthanoid series.

Conclusion: Since the ionic radius does indeed decrease from \(La\) to \(Lu\), Statement B is correct.

Statement C: \(La\) is an element of the transition series rather than the lanthanoid series

Lanthanoids vs. Transition Elements: Lanthanum (\(La\)) is the first element in the lanthanoid series, which consists of 15 elements from \(La\) to \(Lu\). Although it is sometimes considered a transition metal in certain contexts (due to its position in the periodic table), it is primarily classified as a lanthanoid.

Classification: The confusion arises because \(La\) is in the d-block of the periodic table, where transition metals are located, but it is also the first element in the lanthanoid series, making it a borderline case.

Why Statement C Can Be Considered Correct:

Traditional View: While \(La\) is usually classified as a lanthanoid, it can be argued that it shares characteristics with transition metals.

Conclusion: Depending on interpretation, Statement C can be seen as correct, though it is more traditionally classified as a lanthanoid.

Statement D: Atomic radii of \(Zr\) and \(Hf\) are the same because of lanthanoid contraction

Lanthanoid Contraction: The contraction of the lanthanoid series impacts the elements that follow it. Specifically, it causes the atomic radii of hafnium (\(Hf\)) and zirconium (\(Zr\)) to be almost identical, despite \(Hf\) being in the period below \(Zr\).

The additional f-electrons in the lanthanoids shield the outer electrons poorly, resulting in a smaller atomic radius for \(Hf\) than might be expected, bringing it close to that of \(Zr\).

Why Statement D is Correct:

Accurate Explanation: The statement correctly identifies the effect of lanthanoid contraction on the atomic radii of \(Zr\) and \(Hf\).

Conclusion: Since the atomic radii are indeed very similar due to lanthanoid contraction, **Statement D is correct**.

Final Conclusion:

Statement A is the only incorrect statement, as it contradicts the established trend in basicity among lanthanoid hydroxides. Statements B, C, and D are all correct or can be reasonably argued to be correct. Thus, the correct answer is option 3: A only.