# What factor determines electronegativity?

Given the definition of electronegativity, the determining factor is likely nuclear charge (i.e. $Z$, the atomic number).
So, the obvious follow up question is why are, say, sulfur, and chlorine, less electronegative than their first row congeners, oxygen and fluorine respectively. Certainly the 2nd row atoms have greater nuclear charge than the first row. The answer is that when you descend a row, a full or complete shell of electrons effectively shields the valence (the outermost) electrons from the increased nuclear charge. After all these years I think I can still remember Pauling electronegativities of the halogens: F, 4.0; Cl, 3.5; Br, 3.0; I, 2.5; they decrease down a group, but increase across a period (as we would anticipate). These values do have some basis in reality in that Pauling derived them from atomic parameters such as ionization enthalpies of the atoms, but remember they are an $a d$ $h o c$ scale.