Chlorine of fusion and vaporisation of chlorine are again intermediate between those of bromine and fluorine, although all their heats of vaporisation are fairly low (leading to high volatility) thanks to their diatomic molecular structure. The halogens darken in colour as the group is descended: thus, while fluorine is a pale yellow gas, chlorine is distinctly yellow-green. This trend occurs because the wavelengths of visible light absorbed by the halogens increase down the group. Specifically, the colour of a halogen, such as chlorine, results from the electron transition between the highest occupied antibonding "π" molecular orbital and the lowest vacant antibonding
Fluorine perchlorate Fluorine perchlorate Perchloryl Hypofluorite is the rarely encountered chemical compound of fluorine, chlorine, and oxygen with the chemical formula or . It is an extremely unstable gas that explodes spontaneously and has a penetrating odor. One synthesis uses fluorine and perchloric acid, though the action of ClF on water is another method. Another method of synthesis involves the thermal decomposition of tetrafluoroammonium perchlorate, , which yields very pure that may be manipulated and frozen without explosions. Fluorine perchlorate is not analogous to perchloric acid because the fluorine atom does not exist as a positive ion. It contains an oxygen atom