Some years after the discovery of electrons, and still
some years before the discovery of quantum mechanics, G.N. Lewis
at UC Berkeley proposed a theory of the chemical bond. Some of what
he said was wrong, but two of his proposals are still recognized
(and used) for their penetrating insight: the shared electron pair
(covalent) bond, and the valence (Lewis) octet.
These ideas are enshrined in drawings called Lewis
structures. These structures can be drawn quickly and serve
several important uses, from helping us count electrons, to providing
information about electron/bond position, to providing a visual
framework for expressing ideas about chemical reactivity.