Why does fluorine have a high electronegativity?

Fluorine has the highest electronegativity of all elements because of its small atomic size and high effective nuclear charge. Electronegativity is defined as the ability of an atom to attract the shared pair of electrons in a covalent bond toward itself. It's essentially a measure of how strongly an atom pulls on bonding electrons when it forms a chemical bond with another atom. Fluorine's electronegativity value of 4.0 on the Pauling scale makes it the most electronegative element, meaning it has the strongest tendency to attract bonding electrons.

This exceptional electronegativity arises from the combination of two key factors. First, fluorine is the smallest atom in Group 17 and among the smallest in the entire periodic table. This means that bonding electrons are positioned very close to fluorine's nucleus when it forms covalent bonds. Second, fluorine has a high effective nuclear charge, which is the net positive charge experienced by the valence electrons after accounting for shielding by inner electrons. With 9 protons and only 2 inner electrons providing shielding (in the 1s orbital), fluorine's seven valence electrons experience a strong pull from the nucleus with an effective nuclear charge of approximately +7. The combination of this strong nuclear attraction and the minimal distance between the nucleus and bonding electrons results in fluorine's unparalleled ability to attract electrons in chemical bonds. This explains why fluorine forms such polar bonds with nearly every other element and why compounds containing fluorine often exhibit unique chemical properties.