A chloride ion (Cl-) has gained an extra electron compared to a neutral chlorine atom. This means it now has the same number of electrons as a neutral argon atom (Ar).

Here's why:

* Chlorine (Cl) has 17 protons and 17 electrons in its neutral state.

* Chloride ion (Cl-) has gained an extra electron, giving it 18 electrons.

* Argon (Ar) also has 18 electrons.

Therefore, a chloride ion has the same number of electrons as a neutral argon atom.