The alkali metal semiconductor interfaces are currently being investigated by a variety of tools. Most studies to date at half a monolayer coverage have shown preference for either a quasi-hexagonal (H) site or a long-bridge (B) site. At this coverage one-dimensional chain structure for K on Si(001) 2 × 1 have now been confirmed by scanning tunneling microscopy (STM). The data, however, is consistent with either of the two sites. STM investigations at low coverages suggested that alkali metals like K and Cs occupy a novel site, Y, which is a bridge site between two Si atoms belonging to different dimers along the dimer row [110] direction. The total energy calculations for this new Y site, discovered by STM, have shown that it is indeed a site of (local) energy minimum. The ability of the surface silicon atoms, which are not adjacent to the alkali metal atom, to buckle makes the Y site a competitive adsorption site. We deduce the nature of bonding between alkali metals and Si using the STM data. It is concluded that the bond is substantially ionic in nature. © 1992.