In 1958, Allred and Rochow proposed a more reasonable empirical approach to electronegativity based on electrostatic force between the nucleus and electrons. They defined electronegativity as electrostatic force (F) exerted by the effective nuclear charge ( Z_eff )on the outermost electron at a distance r (covalent radius) form the nucleus. That is :
                                                            F ∝ Z_eff
                                                                     r²
Where r = covalent radius of the atom in Å .

Z_eff is calculated according to Slater rules based on screening effects of intervening electrons.
The attractive force(F) is then correlated to Eletrongativity values on the Pauling’s scale using an empirical relationship to obtain Allred-Rochow electronegativites x_AR :
                                                            x_AR = 0.359  Z_eff    + 0.744
                                                                                     r²

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