with a boiling point of 5 °C. Iodine monochloride (ICl) exists as red transparent crystals that 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) is made either by the direct combination of gas-phase astatine with chlorine or by the sequential addition of astatine and dichromate ion to an acidic chloride solution. 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 the direct combination of astatine with either bromine vapour or an aqueous solution of iodine monobromide. Astatine monoiodide (AtI) is made by direct combination of astatine and iodine. No astatine fluorides have been discovered yet. Their absence has been speculatively attributed to the extreme reactivity of such compounds, including the reaction of an initially formed fluoride with the walls of the glass container to form a non-volatile product. Thus, although the synthesis of an astatine fluoride is thought to be possible, it may require a liquid halogen fluoride solvent, as has already been used for the characterization of radon fluorides. In addition, there exist analogous molecules involving pseudohalogens, such as the cyanogen halides. Tetratomic interhalogens Chlorine trifluoride (ClF3) is a colourless gas that condenses to a green liquid, and freezes to a white solid. It is made by reacting chlorine with an excess of fluorine at 250 °C in a nickel tube. It reacts more violently than fluorine, often explosively. The molecule is planar and T-shaped. It is used in the manufacture of uranium hexafluoride. Bromine trifluoride (BrF3) is a yellow-green liquid that conducts electricity — it self-ionises to form [BrF2]+ and [BrF4]−. It reacts with many metals and metal oxides to form similar ionised entities; with other metals, it forms the metal fluoride plus free bromine and oxygen; and with water, it forms hydrofluoric acid and hydrobromic acid. It is used in organic chemistry as a fluorinating agent. It has the same molecular shape as chlorine trifluoride. Iodine trifluoride (IF3) is a yellow solid that decomposes above −28 °C. It can be synthesised from the elements, but care