|Fluorine||F||[He] 2s22p5||Green-yellow gas|
|Chlorine||Cl||[Ne] 3s23p5||Green-yellow gas|
|Bromine||Br||[Ar] 3d104s24p5||Dark red liquid||Iodine||I||[Kr] 4d105s25p5||Grey-black solid|
Generally speaking, the halogens are the most reactive group of non-metals. They are powerful oxidising agents compared with Group 1 and Group 2 metals which are reducing agents. They all have outer electronic configurations of s2p5. Therefore one electron is required to completely fill the p orbital and attain the noble gas configuration. The tendency is to form negatively charged ions.
X + e- X-
Fluorine will most readily accept this electron, i.e. most electronegative, because its outer electrons are nearest to the nucleus; iodine will accept the electron much less readily. Fluorine is therefore the most reactive element in the group. The halogen atoms can form ionic bonds with metals. They can also form covalent bonds with non-metals. The halogens exist as simple diatomic molecules, e.g. Cl2.
The halogens are so reactive they cannot exist free in nature. They are usually found combined with metals. Fluorine is found as fluorspar CaF2 (Blue John) and cryolite Na3AlF6. Chlorine is found as sodium chloride. Bromides and iodides are found in sea water.
Both fluorine and chlorine are obtained by electrolysis. Bromine is obtained from the bromide in sea water by oxidation with chlorine. Chlorine can be made by the electrolysis of a concentrated solution of sodium chloride. The other products are hydrogen and the alkali sodium hydroxide. Three well-established technologies have been established for electrolysing brine:
The half-equations involved are:
|at the anode:||2Cl-(aq) Cl2(g) + 2e-|
|at the cathode:||2H2O(l) + 2e- 2OH-(aq) + H2(g)|
Sodium ions from the sodium chloride are able to combine with the hydroxide ions to form sodium hydroxide.
|E.g.||2Na + Cl2 2NaCl|
|2Fe + 3Cl2 2FeCl3|
|2Al + 3I2 2AlI3|
The halogens are sparingly soluble in water. Chlorine and bromine produce a mixture of acids with water. The solutions act as a bleach.
Cl2(g) + H2O(l) HCl(aq) + HClO(aq)
Iodine is almost insoluble.
The halogens are more soluble in non-polar solvents such as hexane. They produce characteristic coloured solutions, e.g. bromine - orange, iodine - pink.
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