Fluorine is a poisonous pale yellow gas, Chlorine is a poisonous pale green gas, Bromine is a toxic and caustic brown volatile liquid, and Iodine is a shiny black solid which easily sublimes to form a violet vapour on heating.
The elements of Group 7 are a very similar set of non-metals. They all exist as diatomic molecules, X2, and oxidise metals to form Halides. The Halogen oxides are acidic, and the hydrides are covalent. Fluorine is the most electronegative element of all. Electronegativity and oxidising ability reduce on descending the Group. The result of this decreasing electronegativity is increased covalent character in the compounds, so that Aluminium fluoride is ionic whereas Aluminium chloride is covalent.
Occurrence and Extraction
The Halogens are too reactive to occur free in nature. Fluorine is mined as Fluorspar, Calcium fluoride and Cryolite. It is extracted by electrolysis as no oxidant will oxidise fluorides to Fluorine. Chlorine is found in minerals such as Rock Salt, and huge quantities of chloride ions occur in seawater, inland lakes and subterranean brione wells. It is obtained by the electrolysis of molten Sodium chloride or brine. Bromine is also found as the bromide ion in seawater, and in larger quantities in brine wells, from which it is extracted. Iodine is mined as Sodium iodate (V), NaIO3, which is present in Chile saltpetre. It is obtained by reaction with Sodium hydrogensulphite.
At room temeprature all the Halogens occur as diatomic molecules. The melting points, boiling points, atomic radii and ionic radii all increase on descending the Group.
The most characteristic property of the Halogens is their ability to oxidise. Fluorine has the strongest oxidising ability of any element. It is such a strong oxidising agent that it must be prepared by electrolysis. Chlorine is the next strongest oxidising agent within the Group, but it can be prepared by chemical oxidation. Most elements react directly with Chlorine, Bromine and Iodine, with decreasing reactivity going down the Group, but the reaction must usually be initialised with heat or UV light. The oxidation of thiosulphate ion, S2O32-, by the Halogens is quantitative, and so the oxidising agents can be estimated accurately.
Oxides and Oxoacids
There are no Fluorine oxides as Fluorine is more electronegative than Oxygen. Chlorine, Bromine and Iodine each form several oxides, which are thermally unstable, such as Chlorine dioxide, ClO2. The only Fluorine oxoacid, HOF, is unstable at room temperature, but there are many oxoacids of the other Halogens. The best known of these are;