What characteristic distinguishes halogens from other groups of elements?

Halogens are distinctly characterized by having seven valence electrons in their outer electron shell. This arrangement places them in Group 17 of the periodic table. The presence of seven valence electrons means that halogens are just one electron short of achieving a stable octet configuration, which is why they are highly reactive—they readily gain an electron to fill their outer shell, forming negative ions (anions) or covalent bonds with other elements.

This seven-valence-electron configuration is what sets halogens apart from other groups, such as alkali metals, which have one valence electron, or noble gases, which have a complete octet of eight valence electrons and are largely unreactive. Thus, this unique electron configuration is fundamental to understanding the chemical behavior and reactivity of halogens compared to other groups in the periodic table.