The reference standard used for defining atomic mass is

Options

A. H-1
B. C-12
C. C-13
D. C-14

Answer

To solve the question regarding the reference standard used for defining atomic mass, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Atomic Mass**:
- Atomic mass is a measure of the mass of an atom, typically expressed in atomic mass units (AMU).

2. **Identifying the Reference Standard**:
- The atomic mass unit (AMU) is defined based on a specific isotope of an element.

3. **Finding the Isotope Used**:
- The reference standard for atomic mass is defined as 1/12th of the mass of the carbon-12 isotope.

4. **Reason for Choosing Carbon-12**:
- Carbon-12 is chosen because it is the most abundant isotope of carbon in nature, making it a reliable standard for comparison. Other isotopes like carbon-13 and carbon-14 exist, but their natural abundance is lower than that of carbon-12.

5. **Conclusion**:
- Therefore, the reference standard used for defining atomic mass is the carbon-12 isotope.

### Final Answer:
The reference standard used for defining atomic mass is the carbon-12 isotope.