A simple relation exists between electronegativities of cations and their oxidation states and ionic radii. An empirical law is proposed: X = 0.274 z-0.15 z r - 0.01 r+1+${alpha}$, z being oxidation number, r ionic radius in $AA$ and ${alpha}$ a term related to the atomic number. this relation permits to calculate an electronegativity scale covering a large set of electronic and crystallographic situations. An application to the calculation of acid strengths of cations is presented.