Halogens form compounds primarily by gaining one electron to achieve a stable octet configuration. This process is known as gaining an electron or forming an anion.

Here's why:

* Electron Configuration: Halogens have 7 electrons in their outermost shell. By gaining one electron, they complete their octet, becoming more stable.

* Ionic Bonding: The strong attraction between positively charged metal ions and negatively charged halogen ions (halide ions) results in ionic bonds, forming ionic compounds.

* Covalent Bonding: Halogens can also form covalent bonds with other nonmetals, sharing electrons to achieve stability.

For example:

* Sodium chloride (NaCl): Sodium (Na) loses one electron to become Na+, while chlorine (Cl) gains one electron to become Cl-. The electrostatic attraction between these ions forms an ionic bond.

* Chlorine gas (Cl2): Two chlorine atoms share a pair of electrons to form a covalent bond.

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