* Atomic Number: The atomic number of chlorine (17) tells us it has 17 protons.
* Neutral Atom: In a neutral atom, the number of protons (positive charge) equals the number of electrons (negative charge). So a neutral chlorine atom has 17 electrons.
* Gaining an Electron: When chlorine gains an electron, it now has 18 electrons (17 original + 1 gained) and still 17 protons.
* Charge: Since there's one more electron than protons, the chlorine ion will have a negative charge of -1.
Therefore, the charge of a chlorine ion that has gained one electron is -1.
