Daltonian atoms

According to E. Stroker [1], it was not the Democritean atomistic theory of matter which was the precursor of the modem Daltonian atomic theory, as generally accepted, but the Aristotelian concept of minima naturalia, developed in the Middle Age. 

Figure 6. Daltonian atoms and molecules with their surrounding atmospheres of repulsive caloric. (From J. Dalton, A New System of Chemical Philosophy,...

A large part of the success of the hunt for new elements came because of the introduction of new tools such as the spectroscope and electrochemical apparatus. New ideas about the physics of heat also transformed the way scientists thought about materials, as the older fluid models (phlogiston and caloric) were replaced by a kinetic theory of heat. In the kinetic theory, it was the motion of the particles that determined the amount of heat in a substance. As research on electricity, matter, and light (spectra) continued, it became clear that the Daltonian atom model had problems. While it was generally... 

This conclusion is not quite as inevitable as 1 imply here, however. For a different opinion, see, e.g., P. Needham, Has Daltonian Atomism Provided Chemistry with Any Explanations Philosophy ofSdence,l (2004), 1038—1047.A closely related article is P.Needham, Paul. When did Atoms Begin to do any Explanatory Work in Chemistry , International Studies in the Philosophy of Science, 18 (2004), 199-219. Needham s view is based, in part, on that of Pierre Duhem. See Needham s translation of Duhem s article. Atomic Notation and... 

Needham, 2004] P. Needham. Has Daltonian Atomism Provided Chemistry with any Explanations Philosophy of Science, 71, 1038-47, 2004. 

Daltonian atomism promises an understanding of compounds which is more easily grasped than the macroscopic theory of thermodynamics. But Dalton s conception of atoms of the same elemental kind which repelled one another in virtue of a surrounding atmosphere of caloric became an embarrassment which chemists soon gave up. Daltonian atomism came to be understood as an abstraction from what Dalton actually said about the nature of atoms, which its 19th critics chided... 

In offering my version of the origin of Daltons theory, I have generally followed the similarly titled account Leonard Nash, The Origin of Daltons Chemical Atomic Theory, Isis 47 (1956) 101-116, 2 Arnold Thackray, The Origin of Daltons Chemical Atomic Theory Daltonian Doubts Resolved, Isis 57 (1966) 35-55. 

Thackray, Arnold. The Origin of Dalton s Chemical Atomic Theory Daltonian Doubts Resolved. Isis 57 (1966) 35-55. 

My task in this overview lecture is to give you both a feel for this qualitative pre-Daltonian formdation and to properly interface this prehistory with the later developments of the 19th and 20th centuries, which will be the focus of the other talks in this symposium. I hope do this by presenting a very broad overview of how each century tended to focus on a different atomic parameter and how this changing focus was refiected in the chemical thought of the period. 

The classical, or Daltonian, laws of chemical combination depend on strong, primary forces between atoms. Covalency, for instance, nearly always produces discrete molecules, with a well defined structure, and a fixed composition. Only in special cases do the weaker secondary forces give such clear-cut products. The difference is mainly due to the weakness of the forces but it is also due to... 

Radioactivity is contrary to the Daltonian idea of the indivisibihty of atoms. 

Dalton, in line with alchemical practice, represented elements and compounds by symbols—with one important difference. An alchemical svinbol represented the substance in mass, or indefinite quantity whereas Daltonian symbols represented one simple atom or one compound atom of the substance. Some of these symbols arc shown in Fig. 38. 

Later, Berzelius replaced the Daltonian symbols by letters, and here again the letter represented one atom of the element concerned. The modem notation of the symbols shown in Fig. 38 arc H, N, C, O, P, S, for the elements and HO, HN, NO, HC, OC for the compounds. Each of the latter group would now be called a molecular formula, which shows the kind and number of each atom in the molecule which it represents. The corrected molecular formulae for those shown in Fig. 38 arc, in fact HjO (water), NH, (ammonia), NO (nitric oxide) C2H4 (ethylene), and CO (carbon monoxide). 

Gaudin used very clear volume diagrams , with Daltonian symbols for the atoms, to illustrate the applications of Avogadro s hypothesis to the volume relations of reacting gases. E.g. (Fig. 17) ... 

On the basis of this evidence, Rutherford and Soddy in 1902 put forward their disintegration hypothesis. They proposed that radioactive elements were undergoing spontaneous transformation into new elements, and therefore the atoms of radioactive elements were breaking down into new atoms. Within the space of five years the Daltonian concept of the chemical atom had undergone a radical change. Not only had the existence of subatomic particles (electrons) been demonstrated, but also transmutation was shown to be a natural phenomenon. In their paper of 1902 Rutherford and Soddy also demonstrated that the decay of a radioactive substance followed an exponential law. 

Wollaston had been struck with the proportions of oxygen in my table of metallic oxides and he had begun to study the subject when my account of the Daltonian theory prevented him from proceeding with his investigations , the result of which, otherwise, would have been the discovery of the atomic theoiy. It is very doubtful if Wollaston would ever have had the courage to publish this theory, even if it had occurred to him. Wollaston was a personal friend of Berzelius, who greatly admired him. ... 

In this chapter we consider some of the experiments that led to the conclusion that the atom is electrical in nature. Dalton s concept of a structureless atom provided no mechanism to explain these observa tions. These experiments, started over 150 years ago, also culminated in the discovery of X rays and radioactivity (Section 25.1). The spontaneous disintegration of naturally radioactive atoms into smaller particles contradicts the Daltonian hypothesis that atoms are unalterable. In turn, these discoveries inaugurated a more complete theory of the structure of atoms—the nuclear theory of the atom—and reaffirmed the atom as the unit of chemical changes. 

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