  Covalent molecules are characterized by directed bonds, which provide
stability-of-form to the molecules relative atomic positions. In contrast, bulk
metals are characterized by delocalized bonds, where a large number of
resonance structures ensure their high stability. However, reduced to atomic
dimensions, metallic arrangements become increasingly vulnerable to disruptive
entropic fluctuations. Using the smallest possible device, namely, a single
atom held between two atomically sharp probes, force to rupture single-atom
bridges was measured with pico-level resolution, using gold and silver.
Remarkably, measured forces are found to be a precise vector sum (directional
bonding) of cohesive forces between the central and adjacently coordinated
atoms. Over three to four times stronger than bulk, the directional bonds
provide high configurational stability to atomic-sized metallic devices, just
as delocalization-induced resonance stabilization is the emergent response of
bulk metals. Results open new opportunity for molecular electronics without
complications arising from metal/molecule interfaces.
