Here's why:
* Chlorine's electron configuration: Chlorine has 17 electrons with the configuration 2-8-7. This means it has 7 electrons in its outermost shell (valence shell).
* Noble gas configuration: Noble gases are very stable because they have a full outer shell of electrons. The nearest noble gas to chlorine is Argon (Ar), which has a full outer shell of 8 electrons (2-8-8).
* Gaining an electron: To achieve the stable configuration of Argon, chlorine needs to gain one electron. This gives it 8 electrons in its outer shell, making its configuration 2-8-8.
Therefore, by gaining one electron, chlorine becomes a chloride ion (Cl-) and attains the noble gas configuration of Argon.
