On the periodic table, fluorine has the highest electronegativity on the Pauling scale, with a value of 3.98. Electronegativity describes how strongly an atom attracts shared electrons in a chemical bond.
Fluorine is element 9 and belongs to group 17, the halogens. Its atoms are small and have seven valence electrons, so they strongly attract an additional electron to complete their outer shell. This helps explain fluorine’s exceptional reactivity.
Elemental fluorine is a pale yellow, highly toxic gas and is the most reactive of the halogens. It is rarely found free in nature because it reacts readily with other elements. Fluorine compounds, however, are widespread in minerals and industrial materials.
Electronegativity is not the same as electron affinity or ionization energy. Those are related but distinct properties. Pauling’s scale is also a relative scale, originally based on bond energies, rather than a direct measurement with units.