* Protons: A chloride ion (Cl⁻) is formed from a chlorine atom (Cl). Chlorine has 17 protons in its nucleus. The number of protons defines the element, so a chloride ion still has 17 protons.
* Electrons: A neutral chlorine atom also has 17 electrons. To become an ion, a chlorine atom gains one electron. This gives the chloride ion a total of 18 electrons.
In summary:
* Chloride ion (Cl⁻): 17 protons, 18 electrons.
* Neutral chlorine atom (Cl): 17 protons, 17 electrons.
The extra electron is what gives the chloride ion its negative charge.
