Among the following hydroxides which are is most basic ?

Options

A. `Lu(OH)_(3)`
B. `La(OH)_(3)`
C. `Pm(OH)_(3)`
D. `Yb(OH)_(3)`

Answer

To determine which hydroxide among the lanthanoid hydroxides is the most basic, we can follow these steps:

### Step 1: Identify the Hydroxides
The hydroxides in question are:
- La(OH)₃ (Lanthanum hydroxide)
- Pm(OH)₃ (Promethium hydroxide)
- Yb(OH)₃ (Ytterbium hydroxide)
- Lu(OH)₃ (Lutetium hydroxide)

### Step 2: Understand the Trend in Lanthanides
As we move from Lanthanum (La) to Lutetium (Lu) in the lanthanide series, the ionic size of the lanthanide ions decreases. This trend is important because it affects the basicity of their hydroxides.

### Step 3: Analyze the Covalent Character
According to Fajans' rule, the covalent character of a compound increases with a decrease in the size of the cation (the positively charged ion) and an increase in the size of the anion (the negatively charged ion). In our case, the anion (OH⁻) remains the same for all hydroxides.

### Step 4: Determine Basicity
- **Larger Cation (La³⁺)**: La(OH)₃ has a larger cation compared to the others, leading to lower covalent character and higher basicity.
- **Smaller Cation (Lu³⁺)**: Lu(OH)₃ has a smaller cation, leading to higher covalent character and lower basicity.

Thus, as we move from La to Lu, the basicity decreases due to the increase in covalent character.

### Step 5: Conclusion
From the analysis, we conclude that La(OH)₃ is the most basic hydroxide among the given options.

### Final Answer
La(OH)₃ is the most basic hydroxide among the lanthanoids listed.

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