What would cause an atom to have a high electronegativity value?
A complete or nearly complete valence shell orbital about the atom and a high effective nuclear charge on the bonding electrons in question.
The electronegativity value is high when the shielding effect experienced by the electron in question is low and the valence shell of the atom is either full or nearly full.
Electronegativity is a measure of how willing an atom is to attract the electrons present about the bond to itself, the electrons involved in bonding are termed: valence electrons. An atom is less willing to give up valence its electrons when the
The most electronegative atom in the universe is Helium.
Why is Helium extremely electronegative and an element like Cesium, for example not very electronegative?
Think of the electron configuration of Helium and Cesium
Think about all of the electron which separates the valence electron from the nucleus in Cesium, and the lack of separating electron in Helium. This results in the He nucleus having a higher control of its valence electron than does Cesium over its valence electron.