Define isotope and isobar.

Explanation

### Step-by-Step Text Solution:

1. **Definition of Isotope**:
- An isotope is defined as atoms that have the same atomic number (Z) but different mass numbers (A).
- The atomic number represents the number of protons in the nucleus, while the mass number is the total number of protons and neutrons in the nucleus.
- For example, consider the oxygen isotopes:
- Oxygen-15 (O-15) has a mass number of 15.
- Oxygen-16 (O-16) has a mass number of 16.
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