The atomic weights are expressed in terms of atomic mass unit. Which one of the following is used as a standard ?

Options

A. `.^(1)H_(1)`
B. `.^(12)C_(6)`
C. `.^(16)O_(8)`
D. `.^(35)Cl_(17)`

Explanation

To solve the question regarding which standard is used for atomic weights expressed in atomic mass units (AMU), we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Atomic Mass Unit (AMU)**:
- The atomic mass unit (AMU) is a standard unit of mass that quantifies mass on an atomic or molecular scale.

2. **Defining the Standard**:
- The AMU is defined specifically in relation to a particular isotope of carbon.

3. **Identifying the Isotope**:
- The standard used for defining 1 AMU is based on the carbon-12 isotope.

4. **The Definition**:
- 1 AMU is defined as one twelfth (1/12) of the mass of a carbon-12 atom.

5. **Conclusion**:
- Therefore, the standard used for atomic weights expressed in atomic mass units is the carbon-12 isotope.

### Final Answer:
The standard used for atomic weights expressed in atomic mass units (AMU) is the carbon-12 isotope.

---