A hydrogen atom having kinetic energy `E` collides with a stationary hydrogen atom. Assume all motions are taking place along the line of motion of the moving hydrogen atom. For this situation, mark out the correct statement (s).

Options

A. For `E ge 20.4 eV` only, collision would be elastic.
B. For `E ge 20.4 eV` only, collision would be inelastic.
C. For `E = 2.4 eV` only, collision would be perfectly inelastic.
D. For `E = 18 eV` the `KE` of initially moving hydrogen atom after collisition is zero.

Explanation

To solve the problem of a hydrogen atom with kinetic energy `E` colliding with a stationary hydrogen atom, we need to analyze the situation step by step.

### Step 1: Understand the Collision
We have two hydrogen atoms:
- One is moving with kinetic energy `E`.
- The other is stationary.

### Step 2: Determine the Type of Collision
We need to classify the collision as elastic, inelastic, or perfectly inelastic.
- **Elastic Collision**: Total kinetic energy is conserved.
- **Inelastic Collision**: Some kinetic energy is converted into other forms of energy.
- **Perfectly Inelastic Collision**: The two objects stick together after the collision.

### Step 3: Calculate Minimum Energy for Excitation
For hydrogen atoms, the minimum energy required to excite them is 10.2 eV. Since we have two hydrogen atoms, the total minimum energy required to excite both is:
\[ \text{Total Minimum Energy} = 2 \times 10.2 \, \text{eV} = 20.4 \, \text{eV} \]

### Step 4: Analyze the Given Kinetic Energy `E`
- If `E < 20.4 eV`, the collision cannot excite both atoms, and thus it would be an elastic collision.
- If `E = 20.4 eV`, the collision is perfectly inelastic, as it provides just enough energy to excite both atoms.
- If `E > 20.4 eV`, the collision would still be inelastic, as there is excess energy.

### Step 5: Evaluate the Statements
Based on our analysis:
- **Statement A**: If `E > 20.4 eV`, the collision is elastic. (This is incorrect)
- **Statement B**: If `E > 20.4 eV`, the collision is inelastic. (This is correct)
- **Statement C**: If `E = 20.4 eV`, the collision is perfectly inelastic. (This is correct)
- **Statement D**: If `E = 18 eV`, the kinetic energy of the initially moving hydrogen atom after the collision is 0. (This is correct, as it indicates that the moving atom transfers all its energy to the stationary atom)

### Conclusion
The correct statements are B, C, and D.