Problem 73 - A Complete Resource Book in Chemistry for JEE Main

Question

Which statement is not correct? (a) \(\mathrm{La}(\mathrm{OH})_{3}\) is less basic than \(\mathrm{Lu}(\mathrm{OH})_{3}\) (b) in lanthanide series, ionic radius of \(\mathrm{Ln}^{3+}\) ions decreases (c) La is actually an element of transition series (d) atomic radius of \(\mathrm{Zn}\) and Hf are same because of lanthanide contraction

Step-by-Step Solution

Answer

The incorrect statements are (a) and (c).

Step 1: Understand the nature of the compounds

We need to consider the basicity of the hydroxides \(\mathrm{La(OH)}_{3}\) and \(\mathrm{Lu(OH)}_{3}\). Basicity tends to decrease across the lanthanide series due to increased nuclear charge, so \(\mathrm{La(OH)}_{3}\) should be more basic than \(\mathrm{Lu(OH)}_{3}\). Thus, statement (a) is incorrect.

Step 2: Analyze the ionic radius trend

The statement (b) mentions the ionic radius of \(\mathrm{Ln}^{3+}\) ions. In the lanthanide series, the ionic radius does decrease across the series due to the lanthanide contraction. Hence, statement (b) is correct.

Step 3: Check if La is a transition element

Statement (c) asserts that La is a transition element. Lanthanum (La) is often considered the first element of the lanthanide series and is not a part of the transition metals. Hence, statement (c) is incorrect.

Step 4: Compare atomic radii

Statement (d) claims that the atomic radii of Zn and Hf are the same due to lanthanide contraction. This is actually a correct observation; the lanthanide contraction affects the 4d and 5d series, causing elements like Hf to have atomic radii similar to those of elements like Zn. Therefore, statement (d) is correct.

Key Concepts

Ionic radius trend in Lanthanides

In the lanthanide series, the ionic radius of the \(Ln^{3+}\) ions exhibits a significant trend that is quite distinctive. As you move from lanthanum (La) to lutetium (Lu) across the series, the ionic radii decrease. This systematic reduction is known as the "lanthanide contraction."

What drives this contraction? As we progress through the lanthanide series, electrons are added to the 4f orbitals, which are inner shells. These added electrons don't shield each other effectively from the attraction of the nucleus.

Thus, as protons are added to the nucleus without sufficient shielding, the nuclear charge pulls the electron cloud closer, resulting in decreased ionic size.

Key points about lanthanide contraction include:

• The contraction is gradual, affecting each element slightly in series.
• This contraction influences the chemistry of heavier elements found later in the periodic table, as seen with hafnium having similar atomic size to zirconium.

Basicity of Lanthanide Hydroxides

In consideration of the lanthanide hydroxides like \(\mathrm{La(OH)}_{3}\) and \(\mathrm{Lu(OH)}_{3}\), their basicity shows a clear trend across the lanthanide series.

At the beginning of the series, lanthanum hydroxide \(\mathrm{La(OH)}_{3}\) is more basic compared to lutetium hydroxide \(\mathrm{Lu(OH)}_{3}\). This is due to the decreasing ionic size caused by lanthanide contraction, which leads to an increase in the effective nuclear charge on the hydroxide ions.

As a result:

• The electron cloud becomes denser and more tightly held by the nucleus.
• Consequently, the ability of the lanthanide hydroxides to release hydroxide ions in solution decreases.
• This results in lower basicity as you move from La to Lu.

Keep in mind that basicity and electron cloud distribution are interconnected, and this principle emphasizes the unique behaviors of lanthanides.

Characteristics of Transition Elements

Transition elements, forming a crucial part of the periodic table, exhibit unique characteristics that make them stand apart from other elements.

These elements, sitting between groups 3 and 12, typically have partially filled \(d\) or \(f\) orbitals. This electronic configuration results in various interesting properties:

• **Variable Oxidation States:** The availability of \(d\) electrons allows transition elements to exhibit multiple oxidation states.
• **Complex Formation:** Their ability to form stable complexes with ligands is due to vacant \(d\) orbitals that can accept electron pairs.
• **Magnetic Properties:** Many are paramagnetic because of unpaired electrons in the \(d\) orbitals.
• **Catalytic Properties:** As part of chemical reactions, transition metals often serve as catalysts due to their ability to change oxidation states.

The lanthanide contraction we discussed earlier also affects transition metals. For example, elements like hafnium (Hf) have similar atomic radii to zirconium, facilitating the use of these elements in technology and industry.