Visual Elements Periodic Table
Group 17 - The Halogens
The elements of Group 17, the Halogens, are:
All the isotopes of astatine are radioactive, and so this element will not be considered further here.
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 17, the Halogens, 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. Generally, electronegativity and oxidising ability decrease on descending the Group. The result of this decreasing electronegativity is increased covalent character in the compounds, so that AlF3 is ionic whereas AlCl3 is covalent.
Fluorine shows some anomalies because of the small size of its atom and ion. This allows several F atoms to pack around a different central atom, as in AlF63- compared with AlCl4-. The F-F bond is also unexpectedly weak because the small size of the F atom brings the lone pairs closer together than in other halogens, and repulsion weakens the bond.
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 also found in minerals such as rock-salt, and huge quantities of chloride ions occur in seawater, inland lakes and subterranean brine 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 hydrogensulfite.
At room temperature all the halogens exist as diatomic molecules. The melting points, boiling points, atomic radii and ionic radii all increase on descending the Group. The shapes of the covalent molecules and ions are readily explained by VSEPR (valence shell electron pair repulsion) theory and these compounds are often used to illustrate the theory. Fluorine is never surrounded by more than 8 electrons, whereas the other halogens may be surrounded by up to 14 electrons.
The most characteristic chemical feature of the halogens is their ability to oxidise. Fluorine has the strongest oxidising ability, so other elements which combine with fluorine have their highest possible oxidation number. Fluorine is such a strong oxidising agent that it must be prepared by electrolysis. Chlorine is the next strongest oxidising agent, 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 often the reaction must be activated by heat or UV light. The oxidation of thiosulfate ions, S2O 32-, by the halogens is quantitative. This means that oxidising agents can be estimated accurately; the oxidising agent is reacted with excess I- ions, and the liberated I2 titrated with standard thiosulfate solution. The end point is detected with starch as indicator, which forms a dark blue complex with iodine. Chlorine, bromine and iodine disproportionate in the presence of water and alkalis.
Oxides and Oxoacids
There are no fluorine oxides as F is more electronegative than O. 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 salts of these are; hypochlorite, chlorate(I) CIO-, chlorite, chlorate(III) ClO2-, hypochlorate, chlorate(V) CIO3-, perchlorate, chlorate(VII) ClO4- . These are all powerful oxidising agents.
The halogens can combine with each other to form interhalogens and polyhalide ions.
Polyhalide ions have the general formula [Y-X-Y]-. It is not possible for F to represent X in a polyhalide ion as it cannot expand its octet.
Hydrogen halides have the general formula HX. HF is a colourless liquid which boils at 19·5°C, and all the other hydrogen halides are colourless gases. HF is a liquid due to the extensive hydrogen bonding which occurs between molecules. All the hydrogen halides dissolve easily to give acidic solutions, the most widely used being hydrochloric acid, HCl. All except HF are typical acids; they liberate carbon dioxide from carbonates and form salts with basic oxides. HF is a weak acid because the H-F bond is very strong, and because hydrogen-bonding occurs between F- and HF in solution.
The halogens form organic compounds which are best known for their industrial and environmental impact, such as PVC, DDT and TCP.
Oxidation States and Electron Affinities
Fluorine in all its compounds has an assigned oxidation number of -1, as it is the most electronegative of all the elements. The other halogens show a wide range of oxidation numbers, and the redox chemistry of these halogens is important. The oxidation numbers most commonly shown are odd; there are few compounds with even oxidation numbers and they are often thermally unstable. Chlorine is the third most electronegative element after F and O. The halide ions are readily formed by accepting one electron, as this completes an octet of valence electrons. The electron affinity decreases on descending the Group.
The halogens are probably the most important Group of the Periodic Table used in industry. Fluorine is widely used as an oxidising agent. HF is used to etch glass. Chlorine is used for chlorinating drinking water, and in many organochlorine compounds. Some of these, such as the insecticide DDT, are effective but environmentally damaging, and much controversy surrounds their use. Chlorine dioxide is used to bleach wood pulp for paper making, as it gives a good whiteness without degrading the paper. Hypochlorites are used in domestic bleaches. Potassium chlorate(V) is used as an oxidant in fireworks and matches.
For further information look up the individual elements.
Electron Affinity (M-M-)kJ mol-1
Ionisation Energies/kJ mol-1