In aqueous solution, the largest ion is :

Options

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

Answer

To determine the largest ion in aqueous solution among the given options (sodium, cesium, rubidium, and lithium), we need to analyze the ionic sizes of these alkali metals.

### Step-by-Step Solution:

1. **Identify the Alkali Metals**: The given options are all alkali metals from Group 1 of the periodic table:
- Sodium (Na)
- Cesium (Cs)
- Rubidium (Rb)
- Lithium (Li)

2. **Understand Ionic Size Trends**: In Group 1, as we move down the group from lithium to cesium, the atomic size and ionic size increase. This is due to the addition of electron shells, which makes the atoms larger.

3. **Compare the Sizes**:
- Lithium ion (Li⁺) is the smallest because it has the highest effective nuclear charge relative to its size.
- Sodium ion (Na⁺) is larger than Li⁺ but smaller than Rb⁺ and Cs⁺.
- Rubidium ion (Rb⁺) is larger than Na⁺.
- Cesium ion (Cs⁺) is the largest of all the alkali metal ions because it is at the bottom of the group.

4. **Consider Hydration Effects**: In aqueous solutions, the size of the hydrated ion can also be influenced by hydration enthalpy. Smaller ions like Li⁺ have a higher hydration enthalpy, which means they attract more water molecules and can appear larger when hydrated. However, the question asks for the largest ion, not the largest hydrated ion.

5. **Conclusion**: Among the options given, cesium ion (Cs⁺) is the largest ion in aqueous solution due to its position at the bottom of the alkali metal group, which correlates with a larger ionic radius.

### Final Answer:
The largest ion in aqueous solution is **Cesium (Cs⁺)**.

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