Why do nonmetals tend to form negative ions during a chemical reaction?
And that is they tend to accept electron density, and in doing so (i) become reduced, and (ii) cause oxidation in the electron donor....
Dioxygen, and difluorine are the most reactive and oxidizing elements on the Periodic Table; i.e. they have a high nuclear charge that is ineffectively shielded by their valence electrons, and thus their redox potential tends to be high:
And thus non-metals (with the exception of the Noble Gases) have a high effective nuclear charge, that is UNSHIELDED by the valence shell, with the result that they are oxidizing, and thus accept electrons to form anions.........