`H_2` molecule has two electrons and two nuclei. In which form of hydrogen the spin of electrons and also the spin of nuclei are in opposite directions.

Options

A. orthohydrogen
B. parahydrogen
C. metahydrogen
D. `beta`-hydrogen

Answer

To solve the question regarding the form of hydrogen in which the spin of electrons and the spin of nuclei are in opposite directions, we can follow these steps:

### Step 1: Understand the Basics of Hydrogen Molecules
Hydrogen exists primarily in two forms: orthohydrogen and parahydrogen. Each form has distinct properties based on the alignment of the spins of the electrons and nuclei.

**Hint:** Remember that orthohydrogen and parahydrogen are two forms of the H2 molecule, differing in the orientation of spins.

### Step 2: Define Ortho and Para Hydrogen
- **Ortho Hydrogen:** In orthohydrogen, the spins of both electrons and the spins of the nuclei are aligned in the same direction. This means that both the electron spins and the nuclear spins are parallel.

- **Para Hydrogen:** In parahydrogen, the spins of the electrons and the spins of the nuclei are aligned in opposite directions. This means that one electron spin is up while the other is down, and similarly for the nuclei.

**Hint:** Focus on the alignment of spins to differentiate between ortho and para forms.

### Step 3: Identify the Required Condition
The question specifically asks for the form of hydrogen where both the spins of the electrons and the spins of the nuclei are in opposite directions.

**Hint:** Look for the form of hydrogen that has opposite spins for both electrons and nuclei.

### Step 4: Conclusion
Based on the definitions:
- In **orthohydrogen**, the spins are parallel (same direction).
- In **parahydrogen**, the spins are antiparallel (opposite direction).

Thus, the correct answer to the question is **parahydrogen**.

**Final Answer:** The form of hydrogen in which the spin of electrons and the spin of nuclei are in opposite directions is **parahydrogen**.