* It has gained an electron: A neutral chlorine atom has 17 protons and 17 electrons. To become an ion with a 1- charge, it must have gained one extra electron.
* It is now an anion: Anions are negatively charged ions.
* Its electron configuration is now the same as Argon: Chlorine is in group 17 (halogens) and wants to achieve the stable electron configuration of the noble gas Argon (group 18). By gaining an electron, it fills its outer shell and achieves this stable configuration.
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