Diatomic Interhalogens
The interhalogens of form XY have physical properties intermediate between those of the two parent halogens. The covalent bond between the two atoms has some ionic character, the less electronegative element, X, being oxidised and having a partial positive charge. All combinations of F, Cl, Br and I which have the above-mentioned general formula are known, but not all are stable, and some combinations of At with other halogens are not even known.
- Chlorine monofluoride (ClF) is the lightest interhalogen compound. ClF is a colorless gas with a normal boiling point of −100 °C.
- Bromine monofluoride (BrF) has not been obtained as a pure compound — it dissociates into the trifluoride and free bromine.
- Iodine monofluoride (IF) is unstable and decomposes at 0 C, disproportionating into elemental iodine and iodine pentafluoride.
- Bromine monochloride (BrCl) is a red-brown gas with a boiling point of 5 °C.
- Iodine monochloride (ICl) exists as red transparent crystals which melt at 27.2°C to form a choking brownish liquid (similar in appearance and weight to bromine). It reacts with HCl to form the strong acid HICl2. The crystal structure of iodine monochloride consists of puckered zig-zag chains, with strong interactions between the chains.
- Astatine monochloride (AtCl) has been produced by a combination of astatine, perchromate, and chloride, but has not been obtained as a pure compound.
- Iodine monobromide (IBr) is made by the direct combination of the elements to form a dark red crystalline solid. It melts at 42°C and boils at 116°C to form a partially dissociated vapour.
- Astatine monobromide (AtBr) is made by a combination of astatine and an aqueous solution of iodine monobromide.
- Astatine monoiodide (AtI) is made by direct combination of astatine and iodine. It is the heaviest confirmed interhalogen compound.
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