Here's why:

* Chlorine (Cl) is a nonmetal and exists as a neutral atom with 17 electrons.

* To become an ion, chlorine needs to gain one electron to achieve a stable octet (8 electrons) in its outer shell.

* Gaining an electron gives chlorine a negative charge, making it chloride (Cl-).

Chloride ions are essential for many biological processes in the body and are found in various compounds, such as table salt (NaCl).