Which of the following is correct?

Options

A. LiH is thermally more stable than CsH
B. Density of `H_(2)` is about `(1)/(14)`th of that of air
C. Atomic hydrogen is much more reactive than ordinary hydrogen
D. All of these

Answer

To determine which of the given statements is correct, let's analyze each option step by step.

### Step 1: Analyze Option 1
**Statement:** Lithium hydride (LiH) is thermally more stable than cesium hydride (CsH).

- **Reasoning:** Lithium (Li) and cesium (Cs) are both alkali metals in Group 1 of the periodic table. As we move down the group from lithium to cesium, the atomic size increases.
- **Bond Length and Strength:** The bond length between cesium and hydrogen (Cs-H) will be longer than that of lithium and hydrogen (Li-H) due to the larger size of cesium.
- **Conclusion:** Longer bond lengths typically result in weaker bonds. Therefore, LiH, having a shorter bond length, has a stronger bond compared to CsH, making LiH more thermally stable than CsH.

**Verdict:** Option 1 is correct.

### Step 2: Analyze Option 2
**Statement:** The density of H2 is about 1/14th of that of air.

- **Calculation of Density of H2:**
- The molar mass of H2 is 2 g/mol.
- At standard temperature and pressure (STP), 1 mole of any gas occupies 22.4 liters.
- Density of H2 = mass/volume = 2 g / 22.4 L = 0.089 g/L.

- **Calculation of Density of Air:**
- The average molar mass of air is approximately 28.8 g/mol.
- Density of air = mass/volume = 28.8 g / 22.4 L = 1.29 g/L.

- **Comparison:**
- Density of H2 / Density of air = 0.089 g/L / 1.29 g/L ≈ 1/14.4, which can be approximated to 1/14.

**Verdict:** Option 2 is correct.

### Step 3: Analyze Option 3
**Statement:** Atomic hydrogen is much more reactive than ordinary hydrogen.

- **Formation of Atomic Hydrogen:** Atomic hydrogen (H) is produced by the dissociation of molecular hydrogen (H2) at high temperatures (around 3270 K).
- **Reactivity:** Atomic hydrogen is highly reactive because it tends to recombine to form H2. The energy required for this dissociation indicates that atomic hydrogen is more reactive than molecular hydrogen.

**Verdict:** Option 3 is correct.

### Step 4: Conclusion
Since all three options (1, 2, and 3) are correct, the final answer is:

**Answer:** All of the above statements (Option 4) are correct.

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