Which of the following is the least stable?

Options

A. `Pb^(+2) `
B. `Tl^+ `
C. `Al^(+3) `
D. `B^(+1)`

Answer

To determine which of the given elements is the least stable, we need to analyze the oxidation states and their stability based on the inert pair effect. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Elements
First, we need to identify the elements in question. The elements mentioned are lead (Pb) and thallium (Tl), which belong to the carbon and boron groups respectively.

### Step 2: Understand the Group Trends
- **Lead (Pb)**: Lead is in Group 14 (carbon group) with a general electronic configuration of ns² np². It can exhibit oxidation states of +2 and +4. However, due to the inert pair effect, the +2 oxidation state becomes more stable as we move down the group.

- **Thallium (Tl)**: Thallium is in Group 13 (boron group) with a general electronic configuration of ns² np¹. It can exhibit oxidation states of +1 and +3. Again, due to the inert pair effect, the +1 oxidation state becomes more stable as we move down the group.

### Step 3: Analyze the Stability of Oxidation States
- For **Lead (Pb)**: The +2 oxidation state is more stable than the +4 oxidation state due to the inert pair effect.

- For **Thallium (Tl)**: The +1 oxidation state is more stable than the +3 oxidation state due to the inert pair effect.

### Step 4: Compare the Stability
Now, we compare the stability of the least stable oxidation states:
- Lead has a less stable +4 oxidation state.
- Thallium has a less stable +3 oxidation state.

### Step 5: Determine the Least Stable Element
Considering the trends:
- The +4 oxidation state of lead is relatively less stable compared to the +3 oxidation state of thallium.
- Therefore, thallium, with its +3 oxidation state, is less stable than lead in terms of its higher oxidation state.

### Conclusion
Thus, among the elements discussed, **Thallium (Tl)** is the least stable due to the instability of its +3 oxidation state compared to lead's +4 oxidation state.