Define isotopes and give an example

Explanation

### Step-by-Step Solution:

1. **Definition of Isotopes**:
- Isotopes are defined as atoms of the same element that have the same atomic number (Z) but different mass numbers (A). This means that while they have the same number of protons, they have different numbers of neutrons.

2. **Characteristics of Isotopes**:
- Same atomic number (Z): This indicates that they belong to the same element.
- Different mass number (A): This arises from the different number of neutrons present in the nucleus of the atoms.

3. **Example of Isotopes**:
- A common example of isotopes is hydrogen, which has three isotopes:
- **Protium (¹H)**: This isotope has 1 proton and 0 neutrons.
- **Deuterium (²H)**: This isotope has 1 proton and 1 neutron.
- **Tritium (³H)**: This isotope has 1 proton and 2 neutrons.

### Summary:
- Isotopes are atoms of the same element with the same atomic number but different mass numbers. An example is the three isotopes of hydrogen: Protium, Deuterium, and Tritium.