Here's how to figure out the number of electrons in an ionized chlorine atom:

* Neutral Chlorine: A neutral chlorine atom has 17 electrons (its atomic number is 17).

* Ionization: Chlorine tends to gain one electron to become a negatively charged ion (Cl⁻). This is because gaining an electron gives it a full outer shell, making it more stable.

* Total Electrons in Ion: Since chlorine gains one electron when it ionizes, Cl⁻ has 17 + 1 = 18 electrons.