In chemistry, an interhalogen compound is a molecule which contains two or more different halogen atoms (fluorine, chlorine, bromine, iodine, or astatine) and no atoms of elements from any other group. Most interhalogen compounds known are binary (composed of only two distinct elements). Their formulae are generally , where n = 1, 3, 5 or 7, and X is the less electronegative of the two halogens. The value of n in interhalogens is always odd, because of the odd valence of halogens. They are all prone to hydrolysis, and ionize to give rise to polyhalogen ions. Those formed with astatine have a very short half-life due to astatine being intensely radioactive. No interhalogen compounds containing three or more different halogens are definitely known, although a few books claim that and have been obtained, and theoretical studies seem to indicate that some compounds in the series are barely stable. Some interhalogens, such as , , and , are good halogenating agents. is too reactive to generate fluorine. Beyond that, iodine monochloride has several applications, including helping to measure the saturation of fats and oils, and as a catalyst for some reactions. A number of interhalogens, including , are used to form polyhalides. Similar compounds exist with various pseudohalogens, such as the halogen azides (, , , and ) and cyanogen halides (, , , and ). Types of interhalogens 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 halogen, X, being oxidised and having a partial positive charge. All combinations of fluorine, chlorine, bromine, and iodine that have the above-mentioned general formula are known, but not all are stable. Some combinations of astatine with other halogens are not even known, and those that are known are highly unstable. 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. It is created according to the following equation: Br2(l) + F2(g) → 2 BrF(g) Bromine monofluoride dissociates like this: 3 BrF → Br2 + BrF3 Iodine monofluoride (IF) is unstable and decomposes at 0 °C, disproportionating into elemental iodine and iodine pentafluoride. Bromine monochloride (BrCl) is a yellow-brown gas