The Atomic Hypothesis

This hypothesis dates back to Democritus ( 460-370BC) or even earlier to Leucippus (born 475 BC). Democritus was probably the first materialist. He believed that everything, including the soul, was made of atoms... This hypothesis remained dormant for years because it lacked the spiritual component and was atheistic, and hence has never been endorsed (to say the least) by the Church in the western world. 

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During the early years of physieal ehemistry, Ostwald did not believe in the existence of atoms... and yet he was somehow ineluded in the wild army of ionists. He was resolute in his scepticism and in the 1890s he sustained an obscure theory of energetics to take the place of the atomic hypothesis. How ions could be formed in a solution containing no atoms was not altogether clear. Finally, in 1905, when Einstein had shown in rigorous detail how the Brownian motion studied by Perrin could be interpreted in terms of the collision of dust motes with moving molecules (Chapter 3, Section 3.1.1), Ostwald relented and publicly embraced the existence of atoms. 

This chapter is entitled Precursors of Materials Science and the foregoing major Sections have focused on the atomic hypothesis, crystallography, phase equilibria and microstructure, which I have presented as the main supports that made possible the emergence of modern materials. science. In what follows, some other fields of study that made substantial contributions are more brielly discussed. It should be remembered that this is in no way a le.xihnok, my task is not to explain the detailed nature of various phenomena and entitities, but only to outline how they came to be invented or recognised and how they have contributed to the edifice of modern materials science. The reader may well think that I have paid too much attention, up to now, to metals that was inevitable, but I shall do my best to redress the balance in due course. 

Atoms Humbug Rarely does a major new eoneept receive unanimous acceptance. Despite the atomie theory s impact, several major seientists denied the existence of atoms for another eentury. In 1877, Adolf Kolbe, an eminent organic chemisf said [Dalton s atoms are]. .. no more than stupid hallucinations. .. mere table-tapping and supernatural explanations. The influential physicist Ernst Mach believed that scientists should look at facts, not hypothetical entities such as atoms. It was not until 1908 that the famous chemist and outspoken opponent of atomism Wilhelm Ostwald wrote, I am now convinced [by recent] experimental evidence of the discrete or grained nature of matter, which the atomic hypothesis sought in vain for hundreds and thousands of years. (p.45)... 

If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words I believe it is the atomic hypothesis (or the atomic/act, or whatever you wish to call it) that all things are made of atoms—little particles that move around in perpetual... 

All compounds, from the simple ones such as water and ammonia to the most complex, are held together by chemical bonds. All chemical bonds—from purely covalent to strongly ionic—act the way they do because of the nature of the atoms that form the bonds. Our knowledge of those atoms is at the heart of the science of chemistry. Understanding Richard Feynmans little particles has enabled mankind to manage the natural world to suit its needs. Feynman was undoubtedly correct when he said that the atomic hypothesis (or the atomic fact, or whatever you wish to call it) is the most concise and important summary of scientific knowledge produced by mankind. And it is crucial that every generation passes it on to the next. 

Elisabeth Crawford, "Arrhenius, the Atomic Hypothesis, and the 1908 Nobel Prizes in Physics and Chemistry," Isis 75 (1984) 503522. 

On the controversial question of the atomic hypothesis, Edward Frankland was more circumspect ... 

The question whether atoms exist or not.. . belongs rather to metaphysics. In chemistry we have only to decide whether the assumption of atoms is an hypothesis adapted to the explanation of chemical phenomena. . . [and] whether a further development of the atomic hypothesis promises to advance our knowledge of the mechanisms of chemical phenomena.. .. I rather expect that we shall some day find, for what we now call atoms, a mathematico-mechanical explanation, which will render an account of atomic weight, of atomicity, and of numerous other properties of the so-called atoms. 50... 

While divided over the atomic hypothesis, which many chemists deemed metaphysical rather than physical, the entire chemical framework of the nine-... 

Volume 3 explains the systems of molecular and atomic weight, valences, the atomic theory, the system of classification of the elements, and the laws of chemical equilibrium. Here we find Lespieau s view that the goal of chemistry is the formule developee, not the formule brute, and that the atomic hypothesis gives us a striking interpretation and creates a language that is now adopted by all chemists, even those who reject the hypothesis of an indivisible primordial particle.30... 

Lespieau s view was that organic chemistry must make use of physical methods and of physical chemistry and that the experimental work of Graham, Williamson, Wurtz, and, more recently, Raoult had confirmed the superiority of the atomic hypothesis over chemical equivalents. As for the meaning of the constitutional (structural, developed) formula,... 

Most chemists immediately utilized the compositional relationships derived from the atomic hypothesis, but for most of the century they continued to dispute the ontological reality of the atoms that rationalized their useful consequences. Even those most doubtful of the reality of the atom found its operational utility indispensible. Humphry Davy was expressing composition by the relative numbers of proportions by i8io, and William Wollaston attempted a calculus of chemical equivalents in 1814. Jons Jakob Berzelius undertook a systematic determination of the most accurate values to assign each of the atomic weights, publishing his first list in 1813. 

The laws of definite and multiple proportions are also associated with Dalton, for they can be explained by his atomic hypothesis. The law of definite proportions or of constant composition had previously been proposed in the work of Jeremias Richter and Joseph-Louis Proust. The law of multiple proportions came to be regarded as an empirical law quite independent of its relation to the atomic hypothesis or perhaps as an empirical law that inspired the atomic hypothesis however, Roscoe and Harden have shown that in Dalton s mind it was a testable prediction which followed from the atomic hypothesis 4). 

Dulong was also an associate of Berthollet and a member of the Societe d Arcueil. His 1819 paper on heat capacities of elements in collaboration with Alexis-Therese Petit was widely interpreted as support for the atomic hypothesis. They noted that the product of specific heat times atomic weight was very nearly the same for a large number of sohd elements. They recognized that the quantity in qnestion represents the heat capacities of the atoms—or in modem terms, molar heat capacities. And they generalized the resnlts, asserting that, atoms of all simple bodies have exactly the same capacity for heat. (7)... 

Perhaps, the atomic hypothesis will be replaced by another some day— perhaps, but not probably. 

He wrote to Dalton to tell him that this Law of Multiple Proportions was a mystery without the atomic hypothesis. Again Dalton s little spheres had clarified a basic truth. 

Successful model building is at the very heart of modern science. It has been most successful in physics but, with the advent of quantum mechanics, great inroads have been made in the modelling of various chemical properties and phenomena as well, even though it may be difficult, if not impossible, to provide a precise definition of certain qualitative chemical concepts, often very useful ones, such as electronegativity, aromaticity and the like. Nonetheless, all successful models are invariably based on the atomic hypothesis and quantum mechanics. The majority, be they of the ah initio or semiempirical type, is defined via an appropriate non-relativistic, Born-Oppenheimer electronic Hamiltonian on some finite-dimensional subspace of the pertinent Hilbert or Fock space. Consequently, they are most appropriately expressed in terms of the second quantization formalism, or even unitary group formalism (see, e.g. [33]). 

The concept of atoms and molecules emerged in the earliest days of modern chemistry from meticulous measurements of mass and volume. These were the only probes of matter available to early workers. The reality of atoms and molecules was established both by the explanatory power of the atomic model and by experimental evidence of microscopic entities, with Einstein s famous interpretation of Brownian motion in 1905 providing irrefutable confirmation of the atomic hypothesis. 

It is the operational essence of the atomic hypothesis that one can assign properties to atoms and groupings of atoms in molecules and on this basis identify them in a given system or use their properties to predict the behaviour of the system in which they are found. The primary purpose of this section is to demonstrate that the quantum atoms transform this atomic hypothesis into an atomic theory of matter by identifying the atoms of chemistry and defining their properties. This section is not a review of applications, but is rather intended to introduce and illustrate the uses of various atomic properties. 

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