Which one of the following alkaline earth metal ions has the highest ionic mobility in its aqueous solution ?

Options

A. `Be^(2+)`
B. `Mg^(2+)`
C. `Ca^(2+)`
D. `Sr^(2+)`

Explanation

To determine which alkaline earth metal ion has the highest ionic mobility in its aqueous solution, we need to consider the concept of ionic mobility and how it relates to charge density.

### Step-by-Step Solution:

1. **Understanding Ionic Mobility**:
Ionic mobility refers to the speed at which an ion moves through a solution under the influence of an electric field. Higher mobility means the ion can move faster.

2. **Factors Affecting Ionic Mobility**:
- **Charge Density**: This is defined as the charge of the ion divided by its size (radius). Higher charge density leads to stronger interactions with water molecules, which can hinder mobility.
- **Ionic Size**: Smaller ions with higher charge densities will attract more water molecules, creating a larger hydration shell, which reduces their mobility.

3. **Comparison of Alkaline Earth Metal Ions**:
The alkaline earth metal ions we are considering are:
- Beryllium (Be²⁺)
- Magnesium (Mg²⁺)
- Calcium (Ca²⁺)
- Strontium (Sr²⁺)

- **Beryllium (Be²⁺)**: Smallest size, highest charge density, lowest mobility.
- **Magnesium (Mg²⁺)**: Larger than Be²⁺, lower charge density than Be²⁺, higher mobility than Be²⁺.
- **Calcium (Ca²⁺)**: Larger than Mg²⁺, lower charge density than Mg²⁺, higher mobility than Mg²⁺.
- **Strontium (Sr²⁺)**: Largest size, lowest charge density, highest mobility among the four.

4. **Conclusion**:
Since ionic mobility decreases with increasing charge density and size, and Strontium (Sr²⁺) has the largest size and lowest charge density, it will have the highest ionic mobility in aqueous solution compared to the other alkaline earth metal ions.

### Final Answer:
**Strontium ion (Sr²⁺) has the highest ionic mobility in its aqueous solution.**

---