Which of the following pairs represents isobars?

Options

A. `._(19)K^(40)` and `._(11)Na^(23)`
B. `._(2)He^(3)` and `._(2)He^(4)`
C. `._(12)Mg^(24)` and `._(12)Mg^(25)`
D. `._(19)K^(40)` and `._(20)Ca^(40)`

Explanation

To determine which of the following pairs represents isobars, we need to understand the definition of isobars. Isobars are atoms of different elements that have the same atomic mass (mass number) but different atomic numbers.

### Step-by-Step Solution:

1. **Understand the Definition of Isobars**:
- Isobars are nuclei with the same mass number (total number of protons and neutrons) but different atomic numbers (number of protons).

2. **Analyze Each Option**:
- **Option 1**: Potassium (K) with mass number 40 and Sodium (Na) with mass number 23.
- These do not have the same mass number. Therefore, this pair cannot be isobars.
- **Option 2**: Helium (He) with mass number 3 and Helium (He) with mass number 4.
- Both are the same element (Helium), so they cannot be isobars. Isobars must be different elements.
- **Option 3**: Magnesium (Mg) with mass number 24 and Magnesium (Mg) with mass number 25.
- Again, these are both Magnesium, so they cannot be isobars.
- **Option 4**: Potassium (K) with mass number 40 and Calcium (Ca) with mass number 40.
- Both have the same mass number (40) but different atomic numbers (K has atomic number 19 and Ca has atomic number 20). Therefore, this pair is indeed isobars.

3. **Conclusion**:
- The correct answer is **Option 4**: Potassium (K) and Calcium (Ca) are isobars.