Why do atoms of two nonmetals form a compound by sharing valence electrons?

It has to do with their electronegativity. Electronegativity is a measure of how strongly (or weakly) an atom is able to attract electrons. Metals like Na and K have electronegativities that are 1.0 or less. While non-metals have electronegativities like 3 or 4. The larger the number the stronger the atom will attract electrons.

So If Na with an electronegativity of 0.9 comes near F with an electronegativity of 4, F will stip electrons from Na and create an Na+ ion while F will become an F- ion.

HOWEVER, with non-metal to non-metal atoms their electronegativites are so close to each other that no atom will with the "tug of war" for electrons. They end up sharing instead. Everybody is happy.

It depends on electronegativity, in other words how much one atom pulls on another. Electronegativity increases from right to left across the periodic table, and decreases from top to bottom as well. For a compound to have ionic bonds, the difference in electronegativity between two atoms must be at least 1.7, if it is smaller it forms a covalent (sharing valence electrons) bond instead. Since nonmetals are all fairly close to one another on the periodic table, their electronegativities are mostly too close together to form ionic bonds, and instead form covalent ones.

surely Florine isn't a metallic it is in team 17 of the periodic table making it a halogen. Non of the halogens are metals. Florine is a gasoline. this is likely certainly one of the main reactive components via fact it has 7 valence electrons and with a view to grow to be solid it needs 8. the main reactive metallic is francium (Fr) and that's via fact it has one valence electron and with a view to be solid it desires to do away with it. So it varieties ionic bonds with non metals (like fluorine) And while metals "swim" interior the " sea of electrons" it is atoms of a similar metallic next to a minimum of one yet another and because all of them don't have an entire valence shells the elections "leap" from atom to atom. this is likewise why metals are solid conductors of electricity via fact the electrons leap from atom to atom.