We report novel features of potassium deposition on a Si(111)-(2×1) surface as a function of coverage. The binding is ionic even at the saturation coverage without any overlayer metallization. Up to a threshold coverage, the alkali-metal electrons are donated to the empty surface state resulting in a 1D metallic chain. Above this coverage, the conduction-band states are occupied, so that the surface electrons become itinerant leading to the metallization of the substrate and onset of enhanced conductivity. © 1987 The American Physical Society.