Francium Astatide is assumed to be the most dangerous and most heavy salt that would ever exist though it is too unstable to get any amount of it. Because of this, the heaviest salt that is in current use is Cesium Iodide, which is still harmful to ingest, but not nearly as much. It is used as an image intensifier for X-rays. In contrast to this, Hydrogen Fluoride is the lightest salt that is possible, though it is more of a gas than a salt. It is commonly used in industry in the production of florides. The most common salt, Sodium Chloride, is commonly used to season food and is found in seawater. Though Alkali Metals are most commonly used for salt, Alkaline Earth Metals are also used as well. The only catch is that there would need to be two atoms of Halogen to make a stable bond. From this, Radium Astatide becomes the heaviest salt, with this and Francium Astatide being extremely radioactive and not being able to be used in research. This would make Barium Iodide the heaviest used salt, though I could not find any uses for it other than heating with water to form other compounds.