A. Louis Allred and Eugene G. Rochow considered that electronegativity should be related to the charge experienced by an electron on the "surface" of an atom: The higher the charge per unit area of atomic surface the greater the tendency of that atom to attract electrons. The effective nuclear charge, Z, experienced by valence electrons can be estimated using Slaters rules, while the surface area of an atom in a molecule can be taken to be proportional to the square of the covalent radius, r. When r' is expressed in picometres,