The sodium atom easily gets rid of its outer electron. In doing so it becomes a positively charged ion and unveils its second-last electron shell. This shell contains eight electrons, an octet configuration which is very difficult to break up.

    On the other hand, the fluorine atom readily accepts an additional electron on its outer shell; this makes the latter also an eight-electron shell.

    And a negatively charged fluorine ion appears.

    Positive attracts negative. Electrical forces draw the oppositely charged sodium and fluorine ions strongly together. A chemical bond appears between them. This bond is called ionic, and it is one of the principal types of chemical bonds.

    The second is as follows.

    How can such a compound as, say, the fluorine molecule F₂ exist? Fluorine atoms cannot discard electrons from their outer shells. Differently charged ions cannot form in this case.

    The chemical linkage between the fluorine atoms is accomplished by means of a pair of electrons. Each of the atoms deals out one electron to be pooled for common use. As a result, both of the atoms now have eight electrons, as it were, in their outer shells, six of their own and two pooled ones. Such a bond is called covalent.

    The majority of chemical compounds known to us are formed by means of chemical bonds of the first or second type.
 

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