What is the group 17 element with the highest electronegativity?

Fluorine is the Group 17 element with the highest electronegativity, and in fact, it is the most electronegative element in the entire periodic table. Electronegativity refers to how strongly an atom can pull the electrons in a covalent bond toward itself. According to Coulomb's law, this attraction gets stronger when the effective nuclear charge is higher and when the bonding electrons are closer to the nucleus (meaning the atom is smaller).

As you go down Group 17 from fluorine to astatine, electronegativity decreases because the atoms get bigger. All halogens have seven valence electrons and a similar effective nuclear charge of about +7, but each step down the group adds an extra electron shell. This extra shell pushes the valence electrons farther from the nucleus and adds more shielding from inner electrons. Both effects reduce the pull on the shared electrons, so the larger halogens attract bonding pairs less strongly than fluorine does.