Final answer:
Chlorine becomes a negatively charged chloride anion after gaining one electron, fulfilling the octet rule for energy stability.
Step-by-step explanation:
Chlorine needs to gain one electron to become an ion. When a chlorine atom gains an electron, it has one more electron than protons, resulting in a negatively charged chloride anion with a net charge of -1, denoted as Cl-. Chlorine has seven electrons in its outermost shell and needs only one more to fulfill the octet rule and achieve a stable noble gas configuration like argon. This process is termed electron transfer, where chlorine becomes energetically more stable.