The cation which is least stable is

Options

A. `Li^(+)`
B. `K^(+)`
C. `Al^(+2)`
D. `Si^(2+)`

Explanation

To determine which cation is the least stable among the given options, we need to analyze the electronic configurations of the cations formed from Potassium (K), Aluminum (Al), and Silicon (Si).

### Step-by-step Solution:

1. **Identify the Atomic Numbers and Electronic Configurations**:
- **Potassium (K)**: Atomic number = 19
- Electronic configuration: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^1\)
- Cation (K⁺): Loses one electron from the outermost shell.
- Configuration of K⁺: \(1s^2 2s^2 2p^6 3s^2 3p^6\) (completely filled outer shell)

- **Aluminum (Al)**: Atomic number = 13
- Electronic configuration: \(1s^2 2s^2 2p^6 3s^2 3p^1\)
- Cation (Al³⁺): Loses three electrons.
- Configuration of Al³⁺: \(1s^2 2s^2 2p^6\) (completely filled outer shell)

- **Silicon (Si)**: Atomic number = 14
- Electronic configuration: \(1s^2 2s^2 2p^6 3s^2 3p^2\)
- Cation (Si⁴⁺): Loses four electrons.
- Configuration of Si⁴⁺: \(1s^2 2s^2 2p^6\) (completely filled outer shell)

2. **Analyze Stability Based on Electronic Configuration**:
- A cation is generally more stable when its outermost shell is completely filled.
- In the case of K⁺, Al³⁺, and Si⁴⁺, all have completely filled outer shells after ionization.
- However, the stability can also be influenced by the effective nuclear charge and the number of electrons removed.

3. **Evaluate the Stability**:
- Potassium (K⁺) has lost only one electron, maintaining a stable noble gas configuration.
- Aluminum (Al³⁺) has lost three electrons, resulting in a more positive charge and increased instability compared to K⁺.
- Silicon (Si⁴⁺) has lost four electrons, which creates a higher positive charge and further destabilizes the ion.

4. **Conclusion**:
- Among the cations K⁺, Al³⁺, and Si⁴⁺, the cation that is least stable is **Al³⁺** due to the higher positive charge and the fact that it has removed multiple electrons, which increases its instability.

### Final Answer:
The cation which is least stable is **Al³⁺**.