Group 17 elements are known as halogens, with the general electronic configuration n s squared n p to the fifth power. These non-metallic elements exist as diatomic molecules held together by weak van der Waals forces. From fluorine to iodine, we observe increasing molecular size, which leads to stronger intermolecular forces and higher boiling points. This explains why fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid at room temperature.
The reactivity of halogens with hydrogen decreases significantly down Group 17. Fluorine reacts explosively with hydrogen at room temperature, while chlorine requires sunlight or UV light. Bromine needs heating to 200 degrees Celsius with a platinum catalyst, and iodine forms a reversible equilibrium at 400 degrees Celsius. This trend correlates with bond energies: H-F has the highest bond energy at 562 kilojoules per mole, decreasing to only 299 for H-I. As atomic size increases, the H-X bond becomes longer and weaker, making the hydrogen halides less thermally stable.
Chlorine undergoes fascinating disproportionation reactions with sodium hydroxide, where it simultaneously oxidizes and reduces itself. In cold dilute sodium hydroxide, chlorine forms sodium chloride and sodium chlorate one, with chlorine going from oxidation state zero to plus one and minus one. In hot concentrated sodium hydroxide, chlorine forms sodium chlorate five instead, reaching a plus five oxidation state. This occurs because chlorate one ion is unstable at higher temperatures and disproportionates further to form chlorate five and chloride ions.
Halide ions can be systematically identified using silver nitrate followed by ammonia tests. Chloride ions form a white precipitate of silver chloride, which is soluble in both dilute and concentrated ammonia. Bromide ions produce a pale yellow or creamy precipitate of silver bromide, which dissolves only in concentrated ammonia. Iodide ions give a bright yellow precipitate of silver iodide that is completely insoluble in ammonia. This test sequence allows for definitive identification of halide ions in solution.
Halogens have crucial industrial and medical applications. Chlorine is extensively used in water treatment, where it reacts with water to form hypochlorous acid, a powerful disinfectant that kills harmful bacteria and microorganisms. This makes tap water safe for consumption and is also used in swimming pools. In medicine, iodine compounds like povidone-iodine and iodine tincture serve as antiseptics for skin disinfection before surgery and wound treatment. These applications are found in emergency kits and are valuable for water purification during travel.