Which one of the following is not an isotope of hydrogen?

Options

A. Deuterium
B. Para hydrogen
C. Protium
D. Tritium

Explanation

To solve the question "Which one of the following is not an isotope of hydrogen?", we will analyze the given options: Deuterium, Para-hydrogen, Protium, and Tritium.

### Step-by-Step Solution:

1. **Understand the Definition of Isotopes**:
- Isotopes are atoms of the same element that have the same atomic number but different mass numbers. This means they have the same number of protons but a different number of neutrons.

2. **Identify the Isotopes of Hydrogen**:
- Hydrogen has three known isotopes:
- **Protium (¹H)**: Atomic number 1, mass number 1 (1 proton, 0 neutrons).
- **Deuterium (²H)**: Atomic number 1, mass number 2 (1 proton, 1 neutron).
- **Tritium (³H)**: Atomic number 1, mass number 3 (1 proton, 2 neutrons).

3. **Examine the Options**:
- **Protium**: Is an isotope of hydrogen.
- **Deuterium**: Is an isotope of hydrogen.
- **Tritium**: Is an isotope of hydrogen.
- **Para-hydrogen**: This is not an isotope; it refers to a molecular form of hydrogen (H₂) where the spins of the two hydrogen nuclei are aligned in a specific way.

4. **Conclusion**:
- Among the options provided, **Para-hydrogen** is not an isotope of hydrogen.

### Final Answer:
Para-hydrogen is not an isotope of hydrogen.

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