Here's why:
* Neutral chlorine atom: A neutral chlorine atom has 17 protons and 17 electrons. This balances out the positive and negative charges.
* Chlorine ion: Chlorine is highly electronegative, meaning it has a strong tendency to gain electrons. It typically gains one electron to achieve a stable octet configuration in its outer shell. This results in a negatively charged chlorine ion (Cl⁻).
* Electron count: Since it gained one electron, a chlorine ion has 17 protons and 18 electrons.
