"Use Coulomb's law to explain why sodium (Na) is more electronegative than cesium (Cs), even though the nuclear charge on Cs is larger than for Na. To answer this question you need to consider both the effective nuclear charge felt by the valence electrons and the distance of those electrons from the nucleus.
The effective nuclear charge is approximated as the total nuclear charge minus the number of core electrons. Thus, effectively, core electrons decrease the nuclear charge felt by the valence electrons because they are between the nucleus and the valence electrons.