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
Graphite intercalation compound A stage n magnetic GIC has n graphite layers separating successive magnetic layers. As the stage number increases the interaction between spins in successive magnetic layers becomes weaker and 2D magnetic behaviour may arise. Chlorine and bromine reversibly intercalate into graphite. Iodine does not. Fluorine reacts irreversibly. In the case of bromine, the following stoichiometries are known: CBr for n = 8, 12, 14, 16, 20, and 28. Because it forms irreversibly, carbon monofluoride is often not classified as an intercalation compound. It has the formula (CF). It is prepared by reaction of gaseous fluorine with graphitic carbon at 215–230