The mobilities of the alkali metal ions in aqueous solution are `Li^(+) lt Na^(+) lt K^(+) lt Rb^(+) lt Cs^(+) ` because

Options

A. greater is the degree of hydration, lesser is the mobility in aqueous aqueous medium
B. larger the size of cation, greater is the mobility in aqueous medium
C. larger the size of cation, lesser is the mobility of ions in aqueous medium
D. lesser the degree of hydration, lesser is the mobility of ions in aqueous medium

Explanation

To understand the order of mobilities of alkali metal ions in aqueous solution, we need to analyze the factors affecting their mobility. The order given is:

Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺

### Step-by-Step Solution:

1. **Understanding Mobility**:
- Mobility of an ion in solution refers to its ability to move through the solvent (water in this case). Higher mobility means the ion can move more freely.

2. **Size of Alkali Metal Ions**:
- The size of alkali metal ions increases as we move down the group in the periodic table. The order of size is:
- Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺
- Lithium is the smallest ion, while cesium is the largest.

3. **Degree of Hydration**:
- Smaller ions tend to have a higher charge density, which leads to a greater degree of hydration. This means that water molecules surround the smaller ions more closely.
- For example, Li⁺, being the smallest, is surrounded by more water molecules compared to larger ions like Cs⁺.

4. **Impact of Hydration on Mobility**:
- The greater the degree of hydration, the more water molecules are associated with the ion, which hinders its movement through the solution.
- Therefore, Li⁺, with a higher degree of hydration, has lower mobility compared to Na⁺, K⁺, Rb⁺, and Cs⁺.

5. **Order of Mobility**:
- As we go down the group from Li⁺ to Cs⁺, the size of the ions increases, and the degree of hydration decreases. Consequently, the mobility increases.
- Thus, the order of mobility is:
- Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺

6. **Conclusion**:
- The correct explanation for the order of mobilities is that greater the degree of hydration, lesser the mobility in aqueous medium. Therefore, the answer is that the mobility of alkali metal ions in aqueous solution is influenced by their size and degree of hydration.