The alkali metals form salt like hydrides by the direct synthesis at elevated temperature. The termal stability of these hydrides decreases in which of the following orders ?

Options

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

Explanation

To determine the thermal stability of alkali metal hydrides, we need to analyze the trend in stability as we move down the group in the periodic table. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Alkali Metals
The alkali metals include:
- Lithium (Li)
- Sodium (Na)
- Potassium (K)
- Rubidium (Rb)
- Cesium (Cs)

### Step 2: Understand the Formation of Hydrides
When these alkali metals react with hydrogen, they form hydrides:
- Lithium hydride (LiH)
- Sodium hydride (NaH)
- Potassium hydride (KH)
- Rubidium hydride (RbH)
- Cesium hydride (CsH)

### Step 3: Analyze Thermal Stability
Thermal stability refers to the ability of these hydrides to withstand high temperatures without decomposing.

### Step 4: Consider Atomic Size
As we move down the group from lithium to cesium, the size of the alkali metal atoms increases. This increase in atomic size affects the stability of the hydrides:
- Smaller cations (like Li⁺) form stronger bonds with the hydride ion (H⁻) compared to larger cations (like Cs⁺).
- According to Kazan's rule, smaller cations and larger anions lead to stronger compounds.

### Step 5: Establish the Order of Stability
Based on the size and the bond strength:
- Lithium hydride (LiH) will be the most thermally stable due to the small size of Li⁺.
- Sodium hydride (NaH) is less stable than LiH but more stable than the others.
- Potassium hydride (KH) follows NaH in stability.
- Rubidium hydride (RbH) is less stable than KH.
- Cesium hydride (CsH) is the least stable due to the largest size of Cs⁺.

### Final Order of Thermal Stability
Thus, the thermal stability of the alkali metal hydrides decreases in the following order:
**LiH > NaH > KH > RbH > CsH**