Assertion . Among the alkali metals , lithium salts exhibit the least electrical conductance in aqueous solutions .
Reason . Smaller the radius of the hydrated cation , lower is the electrical conductance in aqueous solutions .

Options

A. If both assertion and reason are true, and reason is the true explanation of the assertion.
B. If both assertion and reason are true, but reason is not the true explanation of the assertion.
C. If assertion is true, but reason is false
D. If both assertion and reason are false.

Explanation

To analyze the given assertion and reason, we will break down the concepts step by step.

### Step 1: Understanding the Assertion
The assertion states that among the alkali metals, lithium salts exhibit the least electrical conductance in aqueous solutions.

- **Explanation**: Alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). When these metals form salts and are dissolved in water, they dissociate into cations (Li⁺, Na⁺, K⁺, etc.) and anions. The electrical conductance of these solutions depends on the mobility of the cations in the solution.

### Step 2: Understanding the Reason
The reason provided states that smaller the radius of the hydrated cation, lower is the electrical conductance in aqueous solutions.

- **Explanation**: Hydrated cations are formed when cations are surrounded by water molecules. The size of the hydrated cation affects its mobility; smaller cations are less hindered by water molecules and can move more freely. Therefore, the assertion that smaller hydrated cations lead to lower conductance is incorrect.

### Step 3: Analyzing the Relationship
- **Conductance and Mobility**: The electrical conductance is directly proportional to the mobility of the ions in solution. Since lithium ions (Li⁺) are smaller than other alkali metal ions, they have a higher degree of hydration, which increases their effective size in solution and decreases their mobility.
- **Order of Conductance**: The order of electrical conductance among alkali metal salts in aqueous solution is typically:
- Cs⁺ > Rb⁺ > K⁺ > Na⁺ > Li⁺
This indicates that lithium salts have the least conductance due to the low mobility of Li⁺ ions.

### Step 4: Conclusion
- The assertion is **true**: Lithium salts do exhibit the least electrical conductance in aqueous solutions.
- The reason is **false**: Smaller radius of hydrated cations does not lead to lower conductance; rather, it leads to higher conductance due to increased mobility.

### Final Answer
- The assertion is true, and the reason is false. Therefore, the correct answer is that the assertion is correct, but the reason is incorrect.