We examined the adsorption of Cs on a Si(111)-(7x7) surface by reactive ion scattering (RIS) of hyperthermal Cs+ beams. The RIS from a Cs-adsorbed surface gives rise to Cs-2(+), representing pickup of a surface Cs by a Cs+ projectile. The Cs-2(+) intensity is proportional to the surface coverage of Cs at a high substrate temperature (473 K), while it varies anomalously with the coverage at low temperatures (130-170 K). This observation indicates that RIS selectively detects metallic Cs on the surface, but discriminates ionic Cs. The transition from an ionic to metallic Cs adlayer is driven by the thermal diffusion of Cs and their clustering process.