We present a coverage dependent structural study of Na adsorption on the Si(111)2 x 1 surface by photoemission extended x-ray-absorption fine-structure, core-level, and valence-band photoemission spectroscopies using synchrotron radiation. In the submonolayer coverages, our results indicate the existence of two Na adsorption sites having a large difference in ionicity. Furthermore, a clear ionic to covalent transition is observed when completing the sodium monolayer saturation coverage, which corresponds to the alkali atom adsorbed on a threefold hollow site. These results indicate a very different behavior for the Na adsorption on Si(111)2 x 1 when compared to the Si(100)2 x 1 surface, where the Na-Si bond remains covalent in nature, even at low coverages.