when atoms combine to form molecules with covalent type bonds,the electrons in the bond are shared by both atoms. in the molecules formed, the electrons usually fill all of the available valence orbitals on both of the atoms involved in the bond. For example, in the H-F molecule, we could represent the valence electrons in this molecule with the following formula:
where the dots represent the seven electrons that the fluorine originally had and x represents hydrogen's original electron.
We use different symbols for these electrons only for bookkeeping purposes.