Here's why:

* Chlorine's atomic number is 17. This means it has 17 protons and 17 electrons.

* Electron configuration: The electrons are arranged in shells (also called energy levels) around the nucleus. Chlorine's electron configuration is 2, 8, 7.

* First shell (n=1): Holds a maximum of 2 electrons.

* Second shell (n=2): Holds a maximum of 8 electrons.

* Third shell (n=3): Holds a maximum of 18 electrons, but chlorine only has 7 electrons in this shell.

Therefore, chlorine has three filled electron shells.