Here's why:

* Electron Configuration: Halogens are located in Group 17 (VIIA) of the periodic table. They have 7 valence electrons (electrons in the outermost shell).

* Octet Rule: Atoms tend to gain, lose, or share electrons to achieve a stable electron configuration with 8 valence electrons (octet rule).

* Reactivity: To achieve an octet, halogens readily gain one electron. This makes them highly reactive and prone to forming negative ions called halides.

Example: Chlorine (Cl) has 7 valence electrons. When it gains one electron, it becomes a chloride ion (Cl-) with a full octet of 8 electrons.

Key Point: The tendency to gain an electron is the defining characteristic of halogens as nonmetals.