Which one is least basic ?

Options

A. `Tb(OH)_(3)`
B. `Yb(OH)_(3)`
C. `Gd(OH)_(3)`
D. `Eu(OH)_(3)`

Answer

To determine which hydroxide of the lanthanides is the least basic, we need to analyze the basicity of the hydroxides based on the properties of the lanthanide ions. Here’s a step-by-step solution:

### Step 1: Understand the Lanthanides
The lanthanides are a series of 15 elements from lanthanum (La) to lutetium (Lu) in the periodic table. They are characterized by the filling of the 4f orbital.

### Step 2: Identify the Hydroxides
The question refers to the hydroxides of lanthanides. Common lanthanide hydroxides include:
- La(OH)₃
- Ce(OH)₃
- Yb(OH)₃
- Lu(OH)₃

### Step 3: Analyze Ionic Size and Covalent Character
As we move from La³⁺ to Lu³⁺, the size of the lanthanide cations decreases due to lanthanide contraction. A smaller cation leads to a higher charge density, which increases the covalent character of the hydroxide.

### Step 4: Relate Covalent Character to Basicity
Basicity of hydroxides is inversely related to their covalent character. When the covalent character increases, the basicity decreases. Therefore, the hydroxide of the lanthanide with the smallest cation will be the least basic.

### Step 5: Identify the Smallest Cation
Among the lanthanides, Yb³⁺ (ytterbium) has one of the smallest ionic sizes. Thus, Yb(OH)₃ will have the most covalent character and therefore will be the least basic.

### Conclusion
The least basic hydroxide among the lanthanides is Yb(OH)₃. Therefore, the answer is:

**Option B: Ytterbium hydroxide (Yb(OH)₃) is the least basic.**

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