In the given below alkali metal ions which has lowest ionic mobility in aqueous solutions?

Options

A. `Rb^(+)`
B. `Cs^(+)`
C. `Li^(+)`
D. `Na^(+)`

Answer

To determine which alkali metal ion has the lowest ionic mobility in aqueous solutions, we can analyze the properties of the given ions: lithium (Li⁺), sodium (Na⁺), potassium (K⁺), rubidium (Rb⁺), and cesium (Cs⁺).

### 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. It is influenced by the size of the ion and its interaction with water molecules (hydration).

2. **Comparing Ionic Sizes**: The alkali metal ions increase in size as we move down the group in the periodic table:
- Lithium (Li⁺) is the smallest.
- Sodium (Na⁺) is larger than lithium.
- Potassium (K⁺) is larger than sodium.
- Rubidium (Rb⁺) is larger than potassium.
- Cesium (Cs⁺) is the largest.

3. **Hydration Effect**: Smaller ions tend to have a higher charge density, which leads to stronger interactions with water molecules, resulting in greater hydration. The more hydrated an ion is, the larger its effective size becomes, which can hinder its mobility.

4. **Analyzing Hydration**:
- Li⁺ has the highest hydration energy due to its small size, leading to significant hydration.
- Na⁺, K⁺, Rb⁺, and Cs⁺ follow in order, with Cs⁺ being the least hydrated due to its larger size.

5. **Conclusion**: Since Li⁺ is the smallest and most hydrated, it will have the lowest mobility in aqueous solution. Conversely, Cs⁺, being the largest and least hydrated, will have the highest mobility.

Thus, the alkali metal ion with the lowest ionic mobility in aqueous solutions is **Li⁺ (Lithium ion)**.