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. In the HF molecule all the electrons are indistinguishable. The dot and the x in between the two atoms constitute a pair of bonding electrons. The pair is being shared by both atoms. This shared pair in effect fills the 1s orbital of hydrogen and also gives fluorine eight electrons fill all its valence orbitals. In fact , we count the shared pair twice . This is what we mean when we say the pair is shared. The two electrons involved in this pair have opposite spins. The atoms have combined so as to fill all valence shell orbitals on the two atoms.