Which of the following has the highest tendency to give the reaction,
`M_((g))^(+)overset("aqueous")underset("medium")rarrM_((aq))^(+) +e^-`?

Options

A. Na
B. Li
C. K
D. Rb

Explanation

To determine which of the given alkali metals has the highest tendency to undergo the reaction:

\[ M_{(g)}^{+} \overset{\text{aqueous}}{\underset{\text{medium}}{\rightarrow}} M_{(aq)}^{+} + e^{-} \]

we need to analyze the enthalpy of hydration and the standard electrode potentials of the alkali metals.

### Step 1: Understand the Reaction
The reaction involves the conversion of a gaseous alkali metal ion \( M^{+} \) into its aqueous form \( M_{(aq)}^{+} \) while releasing an electron. This indicates that we are looking for the metal that readily loses an electron and forms a stable aqueous ion.

**Hint:** Consider the properties of alkali metals, particularly their ionization energies and hydration energies.

### Step 2: Analyze the Alkali Metals
The alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). Among these, lithium has the smallest ionic size, which affects its hydration energy.

**Hint:** Remember that smaller ions tend to have higher hydration energies due to stronger interactions with water molecules.

### Step 3: Hydration Energy Trend
The enthalpy of hydration decreases down the group from lithium to cesium. This means that lithium, being the smallest, has the highest enthalpy of hydration, which makes it more favorable for it to exist in the aqueous state.

**Hint:** Think about how the size of the ion influences its ability to interact with water.

### Step 4: Standard Electrode Potential
The standard electrode potentials for the alkali metals show that lithium has the most negative standard electrode potential (E° = -3.04 V). A more negative potential indicates a stronger tendency to lose an electron and thus a stronger reducing agent.

**Hint:** Recall that a more negative electrode potential means a greater tendency to undergo oxidation.

### Step 5: Conclusion
Given that lithium has the highest enthalpy of hydration and the most negative standard electrode potential among the alkali metals, it has the highest tendency to undergo the reaction:

\[ M_{(g)}^{+} \rightarrow M_{(aq)}^{+} + e^{-} \]

Thus, the answer is:

**Lithium (Li)**

**Final Answer:** Lithium (Li)