Here's why:
* Halogens are in Group 17 (VIIA) of the periodic table.
* They have seven valence electrons (electrons in their outermost shell).
* To achieve a stable octet configuration (like noble gases), they need one more electron.
* Therefore, halogens tend to gain one electron to form a negative ion with a charge of -1.
For example, chlorine (Cl) gains one electron to become a chloride ion (Cl⁻).
Let me know if you have any other questions!