An Allegory is used to tell a story that has characters, a setting, as well as other types of symbols, that have both literal and figurative meanings.
Chlorine is counted among this group along with hydrogen (H 2), iodine (I 2), bromine (Br 2), oxygen (O 2), nitrogen (N 2) and fluorine (Fl 2). With seven electrons in its outermost orbit--one electron short of a stable eight-- the element chlorine exists in nature as a diatomic molecule. This arrangement allows two chlorine atoms to share their outermost orbit electrons, achieving stability, compared to the single atom.
If Chlorine can capture an electron from sodium, then that electron will join the 7 to make 8 electrons in the outermost shell. With all electron shells full, chlorine will then be stable. There is so much information in this diagram, it is worthwhile studying carefully to see how much you have grasped.