Ortho- and para-hydrogen differ in

Options

A. atomic number
B. mass number
C. electron spin in two atoms
D. nuclear spin in two atoms

Answer

To solve the question regarding how ortho- and para-hydrogen differ, we can follow these steps:

### Step 1: Understand the Definitions
- **Ortho-hydrogen**: This form of hydrogen consists of two hydrogen nuclei (protons) that have their spins aligned in the same direction.
- **Para-hydrogen**: In this form, the spins of the two hydrogen nuclei are aligned in opposite directions.

### Step 2: Identify the Key Difference
- The primary difference between ortho- and para-hydrogen lies in the **nuclear spin** of the two hydrogen nuclei. In ortho-hydrogen, the spins are parallel (same direction), while in para-hydrogen, the spins are antiparallel (opposite direction).

### Step 3: Analyze the Options
- The question provides four options: atomic number, mass number, electron spin, and nuclear spin.
- Since we have established that the difference is in the nuclear spin, we can eliminate the other options:
- **Atomic number**: Both forms of hydrogen have the same atomic number (1).
- **Mass number**: Both forms have the same mass number (2 for H2).
- **Electron spin**: Hydrogen has no electrons in its molecular form (H2), so this is not applicable.

### Step 4: Conclusion
- Therefore, the correct answer is that ortho- and para-hydrogen differ in their **nuclear spin**.

### Final Answer
The correct option is **D. Nuclear spin of two atoms**.

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