Which elements forms an ion with a 1– charge?

F - Cl - Br - I - At - Ts

Explanation:

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)