Why does the octet rule exist?
Most noble gases have eight valence electrons (He has only 2) which gives them a full outer shell of electrons in the s and p sublevels of their highest energy level.
F forms -1 ions by adding one electron going from
F can add one additional electron by forming a single covalent bond with another atom. This occurs when F atoms bond to each other when forming a diatomic molecule (F-F).
There are elements which are exceptions to the octet rule which do not require 8 valence electrons to be stable. Hydrogen needs only two valence electrons to fill its 1s sublevel, so H atoms form just one covalent bond.