The idea of atomicity, or the definite equivalence of chemical atoms, is the necessary consequence of the theory of replacement. It was employed by Frankland to explain the nature of the organometallic bodies, and its application was further extended by Kolbe to a large number of organic substances.

It is, however, to Kekulé that we owe the complete generalisation of this idea and its systematic application to all classes of compounds. This first rendered it possible to represent, as it is often advantageous to do, the constitution of compounds by completely dissected formulæ.