The covalent bond is a region of relatively high electron density between nuclei which arises at least partly from the sharing of electrons and gives rise to an attractive force and characteristic internuclear distance.
The American chemist Gilbert Newton Lewis postulated the existence of covalent bonds in an effort to explain the way atoms combine with one another and the usual occurrence of stable molecules having an even number of electrons. In his 1916 publication, he states, “…an electron may form a part of a shell of two different atoms and cannot be said to belong to either one exclusively. Hence in general it is impossible to say that one element in a compound has, during chemical change, been oxidised or reduced and that another element has not suffered such a change.”
Here is a photograph of Gilbert Newton Lewis: