I₂, where two iodine atoms share a pair of electrons in order to each achieve a stable octet for themselves; at high temperatures, these diatomic molecules reversibly dissociate a pair of iodine atoms. Similarly, the iodide anion, I^-, is the strongest reducing agent among the stable halogens, being the most easily oxidised back to diatomic I₂. (Astatine goes further, being indeed unstable as At^- and readily oxidised to At⁰ or At⁺.)

The halogens darken in colour as the group