Elements and Atoms: Atoms

By the end of the 18th century, the classical definition of an element as an ultimate product of chemical analysis had become established. This purely operational and macroscopic definition is independent of any theory concerning the nature of matter. By contrast, the modern definition of element is couched in terms of a discrete theory of matter, in terms of atoms. The same reference works consulted on the definition of element all agree when it comes to atom: an atom is the individual structure that constitutes the basic unit of any chemical element.

The term atom has a long history. The concept has evolved from that of an indivisible ultimate particle of matter to a composite structure whose constituent parts themselves have constituent parts. The notion that materials come in discrete packets can be traced to the ancient Greeks. This classical atomism is most frequently associated with Leucippus and Democritus, and it was transmitted to later cultures throught the didactic poem De Rerum Natura by the Roman Lucretius. The atoms of the ancients were literally indivisible (the etymological meaning of atom from the Greek α (not) + τεμνω (cut). The atoms of Newton, so influential in the subsequent development of the concept, were also indestructible, structureless particles [Newton 1704].

The selections of this section will trace only a small part of the story of the atom, focusing on Dalton's hypothesis regarding the chemical significance of atoms and his program for determining one of their salient characteristics (their relative masses). The interaction between Dalton's hypothesis, the subsequent work of Gay-Lussac on combining volumes, and the insights of Avogadro form, in retrospect, a coherent whole, but illustrate, among other things, that progress in science is often non-linear.

This chapter's final selection foreshadows the book's final section, which examines some of the evidence that the atom was not in fact indivisible. Prout's hypotheses were a part of the early 19th-century concept of the atom, but were finally settled only in the first half of the 20th century. In the meantime, the concept of atom developed along distinct chemical and physical lines. "Physical" atoms were a part of theories of matter: were these ultimate particles centers of force, vortices, packets of electrodynamic energy, or something else? In this respect, physicists were ready to look into the structure of the atom before chemists. At the same time, physical manifestations of the discrete nature of matter led to "molecular" theories such as the kinetic theory of gases. Chemists were interested in the units of elements that entered into chemical combinations, often without regard for whether those units were or were not ultimate particles.

Numerous treatments of the history of atoms, of which I mention just a few, consider these and other aspects of the story. Pullman 1998 contains an overarching history of the concept of atom. Van Melsen 1952 concentrates on philosophical and pre-scientific facets of atomism. Rocke 1984 focuses on the chemical concept of atom in the 19th century. Knight 1967 treats scientific concepts of the atom, particularly in the 19th century. Knight 1968 contains facsimiles of many of the original papers mentioned in Knight 1967. Boorse & Motz 1966 is another source of original material and some commentary on the development of scientific atomism.


Back to the top of the table of contents of Elements and Atoms.
Back to the top of Classic Chemistry.