I have previously posted fluorine and chlorine, the two elements at the top of Group VII – the halogens – and astatine near the bottom. Today the two in between: bromine and iodine. These patterns result from their physical properties and give me the rare opportunity to incorporate some organic chemistry.
Similarly to fluorine and chlorine, bromine and iodine have distinctive colours. Bromine is a strong red-brown and is one of only two elements that is liquid at room temperature. The orange-brown solution of bromine in water is useful to distinguish between organic compounds that have a carbon-carbon double bond, as shown, and those that don’t. As the bromine molecules react with the double bonds the solution loses its colour whereas if only single bonds are present, there is no reaction and the colour remains unchanged.
Iodine is a lustrous grey-black solid which sublimes easily, turning directly from the solid to a stunning purple vapour. Relatively few substances sublime as readily (dry ice – solid carbon dioxide – is another) and so I have used the alchemical symbol for sublimation to represent iodine. There is a common misconception that because iodine sublimes so easily, it doesn’t form a liquid at atmospheric pressure at all. In fact it does melt at 114 °C and boils at 184 °C but when heating a sample of the solid, the liquid iodine phase is often obscured by the strongly-coloured purple vapour.