And in fact, fluorine and oxygen are the two most electronegative atoms found on our periodic table. So we just defined what electronegativity is. But where does it come from? What determines electronegativity well, it turns out another important principle known as effective nuclear charge determines or creates electronegativity. Now effective nuclear charge is simply the charge that the balanced electrons found in the outermost shell feel due to the protons found in the nucleus. Remember, electrons and protons attract one another.