An electron can be added to a halogen atom to form a halide ion with
Answer Details
When an electron is added to a halogen atom, it gains one electron to its outermost shell (valence shell) which is now fully occupied, and becomes a negatively charged ion known as a halide ion. Halogens have 7 valence electrons, therefore, the addition of one electron would result in a halide ion with 8 valence electrons, making it more stable. Thus, the correct answer is "8 valence electrons".