Bonding to a quantum corral Chemical bonds generally form between electronic states of atoms; in principle, other electronic states could also form bonds. Stilp et al. found that the electronic states created within quantum corrals, large rings of iron atoms on a copper surface, can form chemical bonds with metal atoms on an atomic force microscope tip. The corral states form from many electrons but have a large spatial extent compared with an atomic orbital. The covalent bond to a 48-atom corral state had an energy of just 5 millielectron volts. Science , abe2600, this issue p. 1196