Question

Which of the following statements is not correct?

• A. La is an element of transition series rather than Lanthanoid series
• B. La(OH)₃ is less basic than Lu(OH)₃
• C. In Lanthanoid series, ionic radius of Ln³⁺ ion decreases
• D. Atomic radii of Zr and Hf are same because of Lanthanoid contraction

Hint:

Lanthanum (La) is often confused with transition metals because it is the first element in the lanthanoid series. However, it is definitely a part of the lanthanoid series and not a transition metal.

Answer 1

The Correct Option is A

Let us evaluate each statement:
- Statement (1): "La is an element of transition series rather than Lanthanoid series." This statement is incorrect. Lanthanum (La) is part of the Lanthanide series, even though it is at the beginning of the series. It is often considered the first element in the lanthanoid series, not a transition element.
Therefore, La is a lanthanoid, not a transition metal.
- Statement (2): "La(OH)₃ is less basic than Lu(OH)₃." This statement is correct. Lanthanum (La) has a larger ionic radius and a lower charge density compared to Lutetium (Lu), making La(OH)₃ a weaker base than Lu(OH)₃. This is because smaller ions like Lu³⁺ have a higher charge density, making their hydroxides more basic.
- Statement (3): "In Lanthanoid series, ionic radius of Ln³⁺ ion decreases." This statement is correct. As we move across the lanthanoid series, the ionic radius of the Ln³⁺ ions decreases due to the increasing nuclear charge, which pulls the electron cloud more tightly, despite the addition of electrons to the 4f orbitals. This phenomenon is called the lanthanoid contraction.
- Statement (4): "Atomic radii of Zr and Hf are same because of Lanthanoid contraction." This statement is correct. The atomic radii of zirconium (Zr) and hafnium (Hf) are nearly the same because the lanthanoid contraction, which causes the reduction in ionic radii in the lanthanide series, also affects the transition metals that follow the lanthanides. Hf is positioned after Zr, and the contraction of the 4f orbitals causes their radii to be nearly identical.
Thus, the incorrect statement is Statement (1).