Hydrochloric acid. Fluorine, bromine, iodine

Fluorine occurs as minerals: fluorspar CaF₂, cryolite Na₃[AlF₆], apatite. Bromine is found in seawater and underground brines, iodine in seaweeds and as iodates (NaIO₃). All halogens are poisonous, sharp-smelling, with diatomic molecules; from F₂ to I₂ the attraction between molecules grows, which is why bromine is a liquid and iodine a solid. Fluorine is the most active: it even decomposes water (2F₂ + 2H₂O → 4HF + O₂) and attacks glass. Br₂ and I₂ are less active; iodine and hydrogen react reversibly (H₂ + I₂ ⇌ 2HI). Halides can be recognised with silver nitrate: AgCl is white, AgBr pale yellow, AgI yellow. Fluorine compounds go into toothpastes that protect teeth from decay and into Teflon; iodine is a trace element needed by humans, part of thyroid hormones, and a lack of it causes goitre, which is why iodized salt is recommended. An alcoholic iodine solution (iodine tincture) disinfects wounds, but use it with an adult’s help and keep it away from small children.