Which of the following statements concerning protium, deuterium and tritium is not true ?

Options

A. They are isotopes of each other
B. They have similar electronic configurations
C. They exist in the nature in the ratio of 1 : 2 : 3 respectively
D. Their mass numbers are in the ratio 1 : 2 : 3 respectively

Answer

To determine which statement concerning protium, deuterium, and tritium is not true, we will analyze each option provided in the question.

### Step 1: Identify the Isotopes
- Protium (¹H) has 1 proton and 0 neutrons, giving it a mass number of 1.
- Deuterium (²H) has 1 proton and 1 neutron, giving it a mass number of 2.
- Tritium (³H) has 1 proton and 2 neutrons, giving it a mass number of 3.

### Step 2: Analyze Each Statement
1. **Statement 1: They are isotopes of each other.**
- This statement is true. Protium, deuterium, and tritium are indeed isotopes of hydrogen, sharing the same atomic number (1) but differing in mass number.

2. **Statement 2: They have similar electronic configuration.**
- This statement is also true. All three isotopes have one electron, leading to the same electronic configuration of 1s¹.

3. **Statement 3: They exist in nature in the ratio 1:2:3 respectively.**
- This statement is not true. In nature, the abundance of these isotopes is approximately 99.956% for protium, 0.0156% for deuterium, and a very trace amount for tritium (around 10^-15%). Therefore, the ratio is not 1:2:3.

4. **Statement 4: Their mass numbers are in the ratio 1:2:3 respectively.**
- This statement is true. The mass numbers of protium, deuterium, and tritium are indeed in the ratio 1:2:3.

### Conclusion
The statement that is not true is **Statement 3: They exist in nature in the ratio 1:2:3 respectively.**

### Final Answer
The correct answer is **Statement 3**.

---