The atom chlorine (Cl) typically forms a -1 charge ion, known as a chloride ion (Cl−).
This occurs because chlorine has seven electrons in its outermost shell and requires one more electron to achieve a stable noble gas configuration, which is eight electrons. When chlorine gains this extra electron, it becomes negatively charged, resulting in the formation of the Cl− ion.
This behavior is a common characteristic of halogens, which are located in Group 17 of the periodic table. By gaining an electron, chlorine enhances its stability and resembles the electron configuration of the inert gas argon. This tendency to gain an electron rather than lose one is why chlorine easily forms a -1 ion in various chemical reactions.