What are isotopes ?

Explanation

### Step-by-Step Solution:

1. **Definition of Isotopes**: Isotopes are defined as atoms that have the same atomic number but different mass numbers.

2. **Understanding Atomic Number and Mass Number**:
- The **atomic number (Z)** is the number of protons in the nucleus of an atom, which determines the element.
- The **mass number (A)** is the total number of protons and neutrons in the nucleus.

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