* Chlorine-35 (³⁵Cl): This is the most abundant isotope, making up about 75.77% of all chlorine atoms. It has 17 protons and 18 neutrons in its nucleus.

* Chlorine-37 (³⁷Cl): This isotope is less abundant, comprising about 24.23% of chlorine atoms. It also has 17 protons, but it has 20 neutrons in its nucleus.

The key difference between these isotopes is the number of neutrons:

* Chlorine-35 has 18 neutrons.

* Chlorine-37 has 20 neutrons.

This difference in neutron count results in a slightly higher atomic mass for Chlorine-37. However, both isotopes have the same number of protons (17), so they have the same chemical properties.

Here's a table summarizing the key differences:

| Isotope | Protons | Neutrons | Atomic Mass | Abundance (%) |

|---|---|---|---|---|

| Chlorine-35 (³⁵Cl) | 17 | 18 | 35.0 u | 75.77 |

| Chlorine-37 (³⁷Cl) | 17 | 20 | 37.0 u | 24.23 |

Importance of isotopes:

* Average atomic mass: The average atomic mass of chlorine (35.45 u) is a weighted average of the masses of its two isotopes, taking into account their relative abundances.

* Nuclear reactions: Isotopes play a crucial role in nuclear reactions, including nuclear fission and fusion.

* Radioactive isotopes: Some isotopes are radioactive, like Chlorine-36. These isotopes are used in various applications, such as radioactive dating and medical imaging.