Which is more electronegative F- or O2- and why?

 

F-: Electronegativity is the measure of how likely an element is to attract a bonding pair of electrons. This trait increases as you get towards the top right of the periodic table. Fluorine is the most electronegative element, with oxygen being second. Fluorine is more electronegative than oxygen since it has more protons (which will attract the pair of electrons due to the opposite charges), and has a smaller radius (because of the attractive force between protons and electrons, having more of both decreases the radius since the atom is being "pulled" closer together)