Here's why:

* Chlorine's Atomic Structure: Chlorine has 17 electrons. It wants to have a full outer shell of 8 electrons, just like the noble gases, which are very stable.

* Gaining an Electron: To achieve this, chlorine will gain one electron, giving it a total of 18 electrons. This makes its outer shell complete.

* Negative Charge: Since it gained an electron (which has a negative charge), chlorine now has one more negative charge than positive charge. This results in a net charge of -1.

Therefore, the chlorine ion, also known as chloride, is represented as Cl⁻.