Chloride

Chloride Ions  are formed from the element chlorine when it gains an electron to form an anion. As shown by this equation:

Cl₂ + 2e^- = 2Cl^-

Chloride ions now have an electronic configuration of [Ne] 3s² 3p⁶

This is a more stable configuration and due to the negative charge, this ion has the ability to be a nucleophile.