Radioactive elements emit `alpha, beta` and `gamma` rays and are characterised by their half-lives. The radioactive isotope of hydrogen is

Options

A. protium
B. deuterium
C. tritium
D. hydronium

Explanation

To solve the question regarding the radioactive isotope of hydrogen, we will follow these steps:

### Step 1: Identify the isotopes of hydrogen
Hydrogen has three isotopes:
- **Protium (¹H)**: This isotope has 1 proton and 0 neutrons. Its mass number is 1.
- **Deuterium (²H)**: This isotope has 1 proton and 1 neutron. Its mass number is 2.
- **Tritium (³H)**: This isotope has 1 proton and 2 neutrons. Its mass number is 3.

### Step 2: Determine which isotopes are radioactive
Out of the three isotopes:
- Protium and Deuterium are stable and non-radioactive.
- Tritium is the only radioactive isotope of hydrogen.

### Step 3: Understand the characteristics of Tritium
Tritium is characterized by its half-life, which is approximately 12.5 years. This means that in 12.5 years, half of a sample of Tritium will have decayed into another element.

### Step 4: Eliminate the incorrect options
- **Protium (Option A)**: Not radioactive.
- **Deuterium (Option B)**: Not radioactive.
- **Hydronium (Option D)**: This is not an isotope; it is an ion (H₃O⁺).

### Conclusion
The radioactive isotope of hydrogen is **Tritium (³H)**, which corresponds to **Option C**.

### Final Answer:
**C. Tritium**

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