Which among `Na,K,Cs` and `Li` forms most stable hydride?

Options

A. LiH
B. KH
C. NaH
D. CsH

Answer

To determine which among Na, K, Cs, and Li forms the most stable hydride, we need to analyze the stability of the hydrides formed by these alkali metals. The hydrides in question are LiH (lithium hydride), NaH (sodium hydride), KH (potassium hydride), and CsH (cesium hydride).

### Step-by-Step Solution:

1. **Identify the Alkali Metals**:
- The elements given are lithium (Li), sodium (Na), potassium (K), and cesium (Cs). All of these belong to Group 1 of the periodic table, known as alkali metals.

2. **Understand Hydride Formation**:
- Alkali metals form hydrides by combining with hydrogen. The stability of these hydrides can vary based on the size of the metal cation.

3. **Analyze Cation Size**:
- As we move down the group from Li to Cs, the size of the alkali metal cations increases. Lithium has the smallest cation size, followed by sodium, potassium, and then cesium, which has the largest cation size.

4. **Stability of Hydrides**:
- The stability of hydrides generally decreases as the size of the cation increases. This is because smaller cations can form stronger bonds with the hydride ion (H⁻) due to effective overlap of orbitals and higher charge density.

5. **Bond Dissociation Energy**:
- The bond dissociation energy of the hydride decreases as the size of the cation increases. This means that the bond in LiH is stronger than in NaH, KH, or CsH.

6. **Conclusion**:
- Since lithium has the smallest cation size and forms the strongest bond with the hydride ion, lithium hydride (LiH) is the most stable hydride among the given options.

### Final Answer:
The most stable hydride among Na, K, Cs, and Li is **LiH (lithium hydride)**.

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