Fluorine, an element in group 7 (also known as group 17 or the halogens) of the periodic table, forms negative ions. Specifically, fluorine atoms gain one electron to achieve a stable electron configuration, becoming negatively charged fluoride ions with a charge of -1. This occurs because fluorine has seven electrons in its outer shell and needs one more electron to complete its octet, resulting in a fluoride ion F−\text{F}^-F−.