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 \(\mathrm{Hf}\) are same because of lanthanide contraction

Step-by-Step Solution

Answer

Statements (a) and (c) are incorrect.

Step 1: Understand the Question

We are required to determine which statement among the four options is incorrect. This involves understanding the properties of lanthanides, transition elements, and the effects of lanthanide contraction.

Step 2: Analyze Statement (a)

The statement claims that \(\mathrm{La(OH)}_3\) is less basic than \(\mathrm{Lu(OH)}_3\). In the lanthanide series, basicity decreases with increase in atomic number due to decreased ionic radii and increased effective nuclear charge. Thus, \(\mathrm{La(OH)}_3\), being at the start of the series, should be more basic than \(\mathrm{Lu(OH)}_3\). Therefore, this statement is incorrect.

Step 3: Analyze Statement (b)

The statement asserts that in the lanthanide series, the ionic radius of \(\mathrm{Ln}^{3+}\) ions decreases. This is due to the lanthanide contraction, where the ionic radii decrease across the series from La to Lu. This statement is correct.

Step 4: Analyze Statement (c)

The statement claims La (lanthanum) is an element of the transition series. La is actually the first element in the lanthanide series, which is separate from the transition series, although it sometimes overlaps in chemical behavior. This statement is incorrect as written.

Step 5: Analyze Statement (d)

The statement suggests that the atomic radii of \(\mathrm{Zn}\) and \(\mathrm{Hf}\) are the same because of lanthanide contraction. Lanthanide contraction affects elements that follow the lanthanides, resulting sometimes in similar sizes to those further up the periodic table, like zinc and hafnium. This statement is essentially true.

Key Concepts

Ionic Radius

Ionic radius refers to the size of an ion, which can vary depending on the number of protons and electrons in the atom. In the lanthanide series, the ionic radius of \(\mathrm{Ln}^{3+}\) ions systematically decreases as you move from lanthanum (La) to lutetium (Lu). This phenomenon is known as "lanthanide contraction."
This decrease is due to the ineffective shielding offered by the 4f electrons, which allows the nucleus to pull the outer electrons closer, reducing the overall size of the ion.

• A smaller ionic radius means the ion is more strongly affected by the chemical environment.
• The consistent decrease in ionic radius impacts bonding and reactivity.

Understanding ionic radius is crucial because it directly influences the strength and character of bonds that an ion can form as well as its stability in compounds.

Basicity of Lanthanides

The basicity of lanthanide hydroxides, such as \(\mathrm{La(OH)}_3\) and \(\mathrm{Lu(OH)}_3\), is an interesting concept in chemistry. Basicity generally refers to the ability of a substance to act as a base, meaning it can accept protons (H\(^+\)). Lanthanide hydroxides become less basic as you move across the series from La to Lu.
The reduction in basicity is attributed to the decreasing ionic radius and the increased effective nuclear charge.

• The smaller the ionic radius, the stronger the interactions between the hydroxide and its environment, reducing its ability to donate pairs of electrons.
• An increased nuclear charge means electrons are held more tightly, further decreasing their ability to act as a base.

This results in \(\mathrm{La(OH)}_3\) being more basic than \(\mathrm{Lu(OH)}_3\), making understanding basicity crucial in predicting reactivity and solubility trends of lanthanides.

Transition Series Elements

Transition series elements, often called transition metals, are those found in the d-block of the periodic table. They are known for their ability to form various oxidation states, colored compounds, and exhibit magnetic properties.
Lanthanum (La), while sometimes associated with transition metals due to similar +3 oxidation states, is actually the first element of the lanthanide series.

• Transition metals have partially filled d orbitals that often lead to high density and high melting points.
• They are critical for many biological processes and industrial applications, such as catalysts.

Understanding the distinction between lanthanides and transition metals is crucial for comprehending their unique roles and properties in chemistry, industries, and nature.