Sea water contains chlorides, bromides and iodides of sodium, potassium, magnesium and calcium, but mainly `NaCl` (`2.5%` by mass). Dried up sea beds contains up to `0.5%` of iodine as sodium iodide (NaI) and chile saltpetre `(NaNO_3)` contains up0 to `0.2%` of sodium iodate `(NaIO_3)`. Hence, sea is the greatest sources of halogens.
