The chemical bonding state of sodium silicates was estimated by a DV-Xα cluster MO method and a population analysis. Cluster models were constructed from the silicate crystals, pure silica, sodium disilicate and sodium metasilicate. The MO calculations successfully reproduced the experimental observations in XPS valence band spectra and core-orbital energies. According to the overlap population, covalent character increased with increasing Na2O content, and it was also the case for Na ions. The exceptional change was found in bridging oxygen (BO), where anti-bonding overlap between Si and BO was clearly seen around the top of the occupied levels and its contribution increased with increasing Na2O content. Correlations between the experimental core-orbital energies and the theoretical electronic populations were also examined, where the best correlation was found in the case of using a total atomic population given by the sum of the net atomic population and the overlap population.