Which has maximum polarising power in cation ?

Options

A. `O^(2-)`
B. `Al^(+3)`
C. `Li^(+)`
D. `Mg^(+2)`

Explanation

To determine which cation has the maximum polarizing power, we can follow these steps:

### Step 1: Understand Polarizing Power
Polarizing power refers to the ability of a cation to distort the electron cloud of an anion. According to Fajan's rule, the polarizing power of a cation is directly proportional to its charge and inversely proportional to its size (or radius).

### Step 2: Identify the Cations
We are given the following cations to compare:
1. Aluminium ion (Al³⁺)
2. Lithium ion (Li⁺)
3. Magnesium ion (Mg²⁺)

### Step 3: Analyze Charge and Size
- **Aluminium ion (Al³⁺)**: Charge = +3
- **Lithium ion (Li⁺)**: Charge = +1
- **Magnesium ion (Mg²⁺)**: Charge = +2

Now, we also need to consider the ionic sizes:
- Al³⁺ has a smaller radius compared to Li⁺ and Mg²⁺ due to its higher charge and the effective nuclear charge pulling the electrons closer.
- Li⁺ has the largest radius among the three because it has the lowest charge.
- Mg²⁺ has a radius smaller than Li⁺ but larger than Al³⁺.

### Step 4: Apply Fajan's Rule
According to Fajan's rule, the polarizing power can be assessed using the formula:
\[ \text{Polarizing Power} \propto \frac{\text{Charge}}{\text{Size}} \]

- For Al³⁺: High charge (+3) and small size = High polarizing power
- For Mg²⁺: Moderate charge (+2) and moderate size = Moderate polarizing power
- For Li⁺: Low charge (+1) and larger size = Low polarizing power

### Step 5: Conclusion
Since Al³⁺ has the highest charge and the smallest size among the given cations, it will have the maximum polarizing power.

### Final Answer
**Aluminium ion (Al³⁺) has the maximum polarizing power among the given cations.**

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