Chemical Compound

The most fundamental change that has taken place in chemistry since the nineteenth century is that atomic theory now permits an understanding of chemical compounds from the particle level rather than from purely empirical data. That is, as our knowledge of atomic structure has grown and developed, our understanding of the reasons that atoms (elements) combine with each other has improved. For example, the question of how and why iron and sulfur combine with each other to form a compound is now approached in terms of how and why an iron atom combines with a sulfur atom to form a molecule of iron(II) sulfide.

A key to the solution of that puzzle was suggested by the German chemist Albrecht Kossel in 1916. In considering the unreactivity of the inert gases, Kossel came to the conclusion that the presence of eight electrons in the outermost energy level of an atom (as is the case with all inert gases) conferred a certain stability on a substance. Perhaps, Kossel said, the tendency of atoms to exchange electrons in such a way as to achieve a full octet (eight) of electrons could explain chemical reactions in which elements combine to form compounds.

Although Kossel had hit on a key concept, he did not fully develop this theory. That work was left to the American chemist Gilbert Newton Lewis. At about the same time that Kossel was proposing his octet theory, Lewis was developing a comprehensive explanation showing how atoms can gain a complete octet either by the gain and loss or by the sharing of pairs of electrons with other atoms. Although Lewis' theory has undergone many transformations, improvements, and extensions (especially in the work of Linus Pauling), his explanation of compound formation still constitutes the heart of such theory today.