Final answer:
Chlorine belongs to the halogens, which is Group 17 on the periodic table, and it becomes a singly negative ion by accepting an extra electron.
Step-by-step explanation:
Chlorine needs one more electron to fill its outer shell. The group to which chlorine belongs is known as the halogens. Halogens are located in Group 17 (Group VII) of the periodic table. These elements, including chlorine, have 5 p electrons in a p5 configuration in their outermost shell, with a vacancy for one more to achieve a stable, noble gas electron configuration. They are highly reactive and tend to form singly negative ions such as Cl- by accepting an extra electron.