Which of the following is triatomic?

Options

A. Carbon dioxide
B. Ammonia
C. Methane
D. Helium

Explanation

To determine which of the given options is triatomic, we need to understand the definition of triatomic and analyze the molecular formulas provided.

### Step-by-Step Solution:

1. **Understand the term "triatomic":**
- Triatomic refers to a molecule that consists of three atoms.

2. **Identify the molecular formulas provided:**
- The question mentions several compounds: CO2 (carbon dioxide), NH3 (ammonia), CH4 (methane), and He (helium).

3. **Count the number of atoms in each compound:**
- **CO2 (Carbon Dioxide):**
- Contains 1 carbon atom and 2 oxygen atoms.
- Total = 1 + 2 = 3 atoms (triatomic).
- **NH3 (Ammonia):**
- Contains 1 nitrogen atom and 3 hydrogen atoms.
- Total = 1 + 3 = 4 atoms (not triatomic).
- **CH4 (Methane):**
- Contains 1 carbon atom and 4 hydrogen atoms.
- Total = 1 + 4 = 5 atoms (not triatomic).
- **He (Helium):**
- Helium is a noble gas and exists as a single atom.
- Total = 1 atom (not triatomic).

4. **Determine which compound is triatomic:**
- From the analysis, CO2 is the only compound that consists of three atoms.

5. **Conclusion:**
- Therefore, the correct answer is CO2 (carbon dioxide), which is triatomic.

### Final Answer:
CO2 (carbon dioxide) is the triatomic molecule.