Concept:Halogens have 7 valence electrons, so they gain one electron to achieve a stable octet.Explanation:A halogen atom, such as fluorine or chlorine, has 7 valence electrons.When it gains one electron, it forms a halide ion with a charge of −1.The added electron completes the outer shell, giving the ion 8 valence electrons.This stable configuration is known as an octet.Answer:A. 8 valence electrons