Bromine structure. The halogens darken in colour as the group is descended: fluorine is a very pale yellow gas, chlorine is greenish-yellow, and bromine is a reddish-brown volatile liquid that melts at −7.2 °C and boils at 58.8 °C. (Iodine is a shiny black solid.) 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 bromine, results from the electron transition between the highest occupied antibonding "π" molecular orbital and the lowest vacant antibonding "σ" molecular orbital. The colour fades at low temperatures, so that solid
Origin and use of the term metalloid their ability to form oxyanions. A common oxyanion of sulfur, for example, is the sulfate ion SO. Many metals can do the same. Chromium, for instance, can form the chromate ion CrO. Berzelius' terminology was widely adopted although it was subsequently regarded by some commentators as counterintuitive, misapplied, incorrect or invalid. In 1825, in a revised German edition of his "Textbook of Chemistry," Berzelius subdivided the metalloids into three classes. These were: constantly gaseous 'gazolyta' (hydrogen, nitrogen, oxygen); real metalloids (sulfur, phosphorus, carbon, boron, silicon); and salt-forming 'halogenia' (fluorine, chlorine, bromine, iodine). In 1844, Jackson gave the meaning of 'metalloid'