Here's why:

* Neutral Chlorine: A chlorine atom has 17 protons (positively charged) and 17 electrons (negatively charged), making it electrically neutral.

* Gaining an Electron: When it gains an electron, it now has 17 protons and 18 electrons.

* Imbalance: This creates an imbalance, with one more electron than protons, resulting in a net negative charge.

So, the chloride ion (Cl⁻) has a negative charge.