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 atom of argon (Ar).

Here's why:

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

* A chloride ion has gained one electron, giving it 18 electrons.

* Argon is a noble gas with 18 protons and 18 electrons.

Atoms and ions with the same number of electrons have the same electron configuration and are considered isoelectronic.