LAUSR

High-resolution images of halogen-containing molecules reveal unusual bonding patterns The halogen group elements (F, Cl, Br, I, and At) are, in the words of the late Oliver Sacks, “the absolute opposite of the inert gases” (1). They are highly electronegative, that is, they strongly attract electrons, making them extremely reactive and even toxic. As a constituent atom in a molecule or complex, halogens therefore tend to be negatively charged. Yet, halogen atoms in molecules can attract other negatively charged species via a noncovalent interaction known as halogen bonding. Somewhat analogous to a hydrogen bond, the halogen bond (2) arises from an emergent electrophilic region on a molecule's constituent halogen atom, allowing it to attract a net negatively charged species on the same molecule or on a neighboring molecule (3). On page 206 of this issue, Han et al. report images of halogen bonding at atomic resolution (4).