Final answer:
Chlorine will gain one electron to achieve a stable noble gas configuration
Step-by-step explanation:
Chlorine has seven valence electrons, one less than the noble gas argon, which has eight valence electrons. Thus, chlorine will gain one electron, forming the anion Cl, and achieving a stable noble gas configuration.
Nonmetals that have close to 8 electrons in their valence shells tend to readily accept electrons to achieve noble gas configuration. Chlorine has the electron configuration of argon when it gains an electron. The resulting ion (Cl) is called chloride ion.
Atoms attempt to get a noble gas electron configuration, which is eight valence electrons. Sodium has one valence electron, so giving it up would result in the same electron configuration as neon. Chlorine has seven valence electrons, so if it takes one it will have eight (an octet). Chlorine has the electron configuration of argon when it gains an electron.