Ionicity of simple binary oxides

Although for most elements the Pauling electronegativity varies only slightly from one compound to another (assuming constant oxidation number), for oxygen there is enormous variation. For binary oxides, MyOz, where the oxidation number of M corresponds to a noble-gas electronic configuration, e.g. as in Na2O, MgO or P2O5 but not As2O3 or MnO, the electronegativity of oxygen, xo, depends upon the electronegativity of M, xM, according to: xo= 4.1 –xM–0.25//0.86. This relationship is shown to be in accordance with electronegativity, xo, decreasing as the negative charge on oxygen increases. It also accounts for smaller than expected electronegativity differences, xo–xM, when M is a highly electropositive element such as an alkali metal.