* Chlorine's Atomic Number: Chlorine (Cl) has an atomic number of 17. This means a neutral chlorine atom has 17 protons and 17 electrons.

* Formation of Chloride Ion: A chlorine atom gains one electron to become a chloride ion (Cl⁻). This is because gaining an electron gives it a full outer shell of electrons, making it more stable.

* Total Electrons in Chloride Ion: Since the chlorine atom gains one electron, a chloride ion has 17 + 1 = 18 electrons.