Hydrogen occurs in nature in three different isotopic forms-protium, deuterium and __________

Explanation

To answer the question about the three isotopic forms of hydrogen, we can follow these steps:

### Step-by-Step Solution:

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

2. **Identify the Isotopes of Hydrogen**: Hydrogen has three isotopes:
- **Protium (¹H¹)**: This isotope has 1 proton and 0 neutrons. It is the most abundant form of hydrogen.
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