All the halogens, are the elements that forms a ion with a -1 charge.
The elements from group 17 in the Periodic Table are the one that has 7 electrons of valence. They need only 1 e⁻ to have the closest noble gas conformation so, they can comply with the octet rule.
This is an example:
Cl₂ (g) + 2e⁻ → 2Cl⁻
Chlorine which is a diatomic gas, gains 2 electrons to make 2 moles of chloride (in order to the balance the reaction)
