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How does gaining one electron give a halogen atom a stable electron arrangement?
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halogen
Modern version of the periodic table of the elements.
Halogen atoms possess seven valence electrons in their outermost electron shell. Because this shell is one electron short of a full complement, halogens are highly reactive and tend to acquire an additional electron from other atoms.12
By gaining this single electron, a halogen atom achieves the same electron configuration as the noble gas that immediately follows it in the periodic table. This resulting arrangement is characterized by a full outer shell—often referred to as an octet—which is exceptionally stable. This stability explains why halogens readily form anions and why they are such effective oxidizing agents.1234