Here's why:
* Halogens are in Group 17 (VIIA) of the periodic table. This means they have 7 valence electrons (electrons in their outermost shell).
* They are highly electronegative. This means they have a strong tendency to attract electrons.
* To achieve a stable octet (8 valence electrons) they need to gain one electron.
Therefore, they become negatively charged ions with a charge of -1 (halide ions).
Examples:
* Chlorine (Cl) gains an electron to become chloride ion (Cl-)
* Bromine (Br) gains an electron to become bromide ion (Br-)
* Fluorine (F) gains an electron to become fluoride ion (F-)
