Alkali metals react with water vigorously to form hydroxides and dihydrogen. Which of the following alkali metals reacts with water least vigorously ?

Options

A. `Li`
B. `Na`
C. K
D. `Cs`

Explanation

To determine which alkali metal reacts with water least vigorously, we need to consider the reactivity trends of alkali metals and their interaction with water.

### Step-by-Step Solution:

1. **Understanding Alkali Metals**: Alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). They are known for their vigorous reactions with water, producing hydroxides and hydrogen gas.

2. **Reactivity Trend**: The reactivity of alkali metals with water increases as you move down the group in the periodic table. This is due to the decreasing ionization energy, which makes it easier for the larger atoms to lose their outermost electron.

3. **Hydration Enthalpy**: Hydration enthalpy refers to the energy released when ions are surrounded by water molecules. It decreases down the group as the size of the alkali metal ions increases. Smaller ions like lithium have higher hydration enthalpy compared to larger ions like cesium.

4. **Analyzing Lithium's Reactivity**: Although lithium is at the top of the group and generally expected to react vigorously, its small size makes it less reactive with water compared to other alkali metals. The small size of lithium results in stronger interactions with water molecules, making it more difficult for lithium to react vigorously.

5. **Conclusion**: Based on the above analysis, lithium (Li) is the alkali metal that reacts with water least vigorously compared to sodium (Na), potassium (K), and others.

### Final Answer:
**Lithium (Li)** reacts with water least vigorously among the alkali metals.