Experimental data are now available on the structure of heavy alkalis, and especially Cs, in highly expanded forms. First, the neutron diffraction experiments of Hensel and co-workers on expanded fluid Cs taken up the liquid-vapor coexistence curve toward the critical point show a dramatic decrease in coordination number but a relatively minor increase in the nearest-neighbor Cs-Cs bond length. Less extensive data for liquid Rb exhibit similar features. Subsequently, by deposition on semiconductor substrates, and in particular GaAs and InSb, one- and two-dimensional ordered structures of Cs have been established, with near-neighbor Cs distances substantially longer than in bulk Cs and being governed by the geometry of the substrate. These experimental results and their relation to electrical conductivity are here analyzed in terms of a localized chemical bonding picture.