Iodine 1% of a sample of gaseous iodine at atmospheric pressure is dissociated into iodine atoms at 575 °C. Temperatures greater than 750 °C are required for fluorine, chlorine, and bromine to dissociate to a similar extent. Most bonds to iodine are weaker than the analogous bonds to the lighter halogens. Gaseous iodine is composed of I molecules with an I–I bond length of 266.6 pm. The I–I bond is one of the longest single bonds known. It is even longer (271.5 pm) in solid orthorhombic crystalline iodine, which has the same crystal structure as chlorine and bromine. (The record is
Chemistry: A Volatile History all the elements in their order: Notice carbon and silicon are in Group IV and the volatile gases fluorine, chlorine and bromine are in Group VII. Mendeleev was sufficiently confident in the layout of his table that he was willing to leave gaps for unknown elements to make the pattern fit – believing other elements would later be discovered that filled the gaps. So, for Mendeleev to be vindicated, the gaps needed to be filled, and luckily, in 1859, new instrumentation had been developed for discovering elements. Robert Bunsen knew that when certain elements burned in the flames of his