Electronegativity (EN) is the ability of an atom or molecule to attract electron towards its nucleus, which is relative to the atom or molecule to which it is bonded or is attracted to. Fluorine has the highest electronegativity in the entire periodic table with EN value of 3.98; this means that out of 4 bonded electrons 3.98 fraction of it is shared with fluorine. Thus the ability to attract electron from other elements is more pronounced than the atom with which it is bonded. And as fluorine has the highest reduction potential (E°_cell =2.87) in the spectrochemical series, it cannot undergo oxidation itself. Thus cannot display disproportionation reactions.