Sulfur, with the atomic number 16, has six electrons in its outermost shell (the third energy level). To achieve a stable electron configuration similar to that of the noble gas argon, sulfur needs to gain two additional electrons. Therefore, when sulfur forms an ion, it typically gains two electrons, resulting in a negatively charged ion known as a sulfide ion (S2-).
This process is essential in various chemical reactions, particularly in forming salts where sulfur can combine with metals. The ability to gain electrons is a key characteristic of nonmetals like sulfur, allowing them to achieve stability through the octet rule.