The trend of basicity of lanthanoid hydroxides

Options

A. increases across the lanthanoid series
B. decreases across the lanthanoid series
C. first increases and then decreases
D. first decreases and then increases.

Explanation

To determine the trend of basicity of lanthanoid hydroxides, we need to analyze the properties of lanthanide elements and their hydroxides. Here’s a step-by-step solution:

### Step 1: Understand the Lanthanide Series
The lanthanide series consists of 15 elements from lanthanum (La) to lutetium (Lu). These elements are known for their similar chemical properties and are often referred to as f-block elements.

**Hint:** Remember that the lanthanide series includes elements with atomic numbers from 57 (La) to 71 (Lu).

### Step 2: Analyze the Hydroxides of Lanthanides
Lanthanoid hydroxides can be represented as Ln(OH)₃, where Ln represents the lanthanide ion. The basicity of these hydroxides is influenced by the size and charge of the lanthanide ions.

**Hint:** Hydroxides are basic compounds, and their basicity can be influenced by the cation's properties.

### Step 3: Consider the Ionic Size
As we move across the lanthanide series from La to Lu, the ionic size of the lanthanide ions decreases. This is due to the increasing nuclear charge which pulls the electrons closer to the nucleus, resulting in a smaller ionic radius.

**Hint:** The trend in ionic size is crucial for understanding the basicity of hydroxides.

### Step 4: Relate Ionic Size to Basicity
The basicity of hydroxides is generally related to the size of the cation. Larger cations tend to stabilize the hydroxide ion (OH⁻) better than smaller cations. Therefore, as the size of the lanthanide cations decreases across the series, the basicity of their hydroxides also decreases.

**Hint:** Think about how a larger cation can better accommodate the hydroxide ion compared to a smaller one.

### Step 5: Conclusion on Basicity Trend
Based on the analysis, the basicity of lanthanoid hydroxides decreases as we move from lanthanum to lutetium. Therefore, the correct answer to the question regarding the trend of basicity of lanthanoid hydroxides is that it decreases across the lanthanide series.

**Final Answer:** The basicity of lanthanoid hydroxides decreases across the lanthanide series.