Answer is: electronegativity decreases with increase of atomic radius, because of increased distance between the valence electrons and nucleus, so atoms tends to lose electrons easier.
Electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.
The atomic radius of a chemical element is a measure of the size of its atom.The atomic radius varies with increasing atomic number, but usually increases because of increasing of number of electrons.The atomic radius decreases across the periods because an increasing number of protons, because greater attraction between the protons and electrons.