The symbol for a chlorine ion that has ionized to have a stable electron configuration is Cl⁻.

Here's why:

* Chlorine (Cl) has 17 electrons.

* To achieve a stable electron configuration, chlorine needs to gain one electron to have the same electron configuration as Argon (18 electrons), which is a noble gas.

* When chlorine gains an electron, it becomes negatively charged because it now has one more electron than protons. This is represented by the superscript "-" in the symbol Cl⁻.