Weak reductant in alkali metal is-

Options

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

Answer

To determine the weak reductant among alkali metals, we need to analyze the properties of alkali metals and their reducing abilities.

### Step-by-Step Solution:

1. **Understanding Reductants**: A reductant (or reducing agent) is a substance that donates electrons in a chemical reaction and gets oxidized in the process. In the context of alkali metals, we are looking for the one that has the weakest ability to donate electrons.

2. **Ionization Energy Trend**: Alkali metals have a trend in ionization energy that decreases as we move down the group from lithium (Li) to cesium (Cs). This means that it becomes easier to remove an electron from the outer shell as we go down the group.

3. **Comparing Alkali Metals**: The alkali metals in order of increasing atomic number are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). As we move down the group, the reducing power generally increases because the ionization energy decreases.

4. **Hydration Enthalpy Consideration**: Lithium has a high hydration enthalpy due to its small size, which compensates for its high ionization energy. This means that while lithium has a high ionization energy, its strong hydration energy makes it a strong reductant.

5. **Identifying the Weakest Reductant**: Given that lithium has the highest ionization energy and its hydration enthalpy compensates for it, it behaves differently compared to other alkali metals. Sodium, on the other hand, has a lower ionization energy and does not have the same level of hydration enthalpy as lithium, making it a stronger reductant than lithium.

6. **Conclusion**: Therefore, among the alkali metals, lithium is identified as the weak reductant due to its high ionization energy and significant hydration enthalpy, which limits its reducing ability compared to sodium and other alkali metals.

### Final Answer:
The weak reductant in alkali metals is **lithium (Li)**.