HISTORY

The History of the Particle Theory




Two particle theories feature in chemistry textbooks - Dalton's atomic theory and the molecular kinetic theory; however, they rarely appear together and the account of each is more descriptive than explanatory. Dalton's atomic theory and the molecular kinetic theory are usually presented using some of the postulates in this list (Garnett, 1996; Bucat, 1984).





Matter consists of submicroscopic, indestructible particles called atoms.





All atoms of an element are identical and have the same mass but atoms of different elements have different masses.





Particles join together in simple consistent ratios when two different substances react to form a third substance.





Mass is conserved in these reactions.





Gas particles are evenly scattered in an enclosed space and there are empty space between particles.





Gas particles are in constant random motion and collisions are perfectly elastic.





Particles move slower in liquids and vibrate about fixed positions in solids.





The spacing between solid-solid, liquid-liquid and gas-gas particles is close to 1:1:10 (Andersson, 1990; de Vos and Verdonk, 1996)



The first postulate is intuitive (matter comprises tiny indivisible particles called atoms) but the remainder are counterintuitive and abstract (e.g., empty spaces separate particles; particles are in constant random motion). Secondary students find this theory difficult to mentally model. Postulate 8 is not discussed in many textbooks and, when it is, the spacing is mostly incorrect (Wilbraham et al., 1997).



History of the Atomic Theory



The following account argues that there are good knowledge and epistemological reasons for retracing the history of the atomic concept. The atomic and kinetic theories grew side-by-side from the rigorous investigations of the 'pneumatic' chemists (e.g., Boyle, Gay-Lussac and Avogadro) and the 'mass' chemists (e.g., Lavoisier, Proust, and Dalton). The atomic theory enunciated by Dalton was revolutionary because he proposed a causal particle explanation for chemical reactions. He explained that reacting masses combine in repeatable, simple ratios because the mass-ratios of reactants and products are the macroscopic manifestation of simple rearrangements of invisible and independent particles. Dalton's theory was powerful because it included a causal explanation that agreed with the available evidence and made predictions that could be tested and falsified. He argued his theory of particle action in 1803-8 yet the full acceptance of the atomic theory and Avogadro's Law took almost 50 years. In this period, chemists argued for and against atoms and Ostwald maintained his objection to the atomic theory well into the 20th Century. If great chemists had problems with atoms and molecules, why do we diminish the problems students face?



The delay was almost entirely due to the dominance of Dalton's "rule of greatest simplicity" and Dalton's insistence that gas particles differ significantly in size, meaning that equal volumes of gas under the same conditions of temperature and pressure do not contain equal numbers of particles (Nash, 1957). In 1808, Gay-Lussac showed that when hydrogen and oxygen react to form water vapour, the combining volumes (and the products under the same conditions) are simple whole number ratios. These data mirrored Dalton's findings but Gay-Lussac's data were more accurate and precise and, when repeated and interpreted by Avogadro in 1811, led Avogadro to assert that equal volumes of gas at the same temperature and pressure do contain equal number of particles and that oxygen and hydrogen particles are diatomic molecules. Dalton's insistence that oxygen and hydrogen particles are single atoms inhibited the full realization of the importance of Gay-Lussac's and Avogadro's ideas until 1860 and stalled the progress of his own atomic theory.



Dalton's "rule of greatest simplicity" insisted that when two elements, A and B combined to form just one new substance, the combining ratio is 1:1 or AB unless there are very good reasons for a different ratio. The rule can be understood as an application of Occam's Razor where the simplest explanation, the one that makes the least assumptions, is deemed best. The rule went on to say that if two compounds resulted from A + B, then the compounds should be AB and A2B or AB2; if three are possible then the additional formula should be AB3 or A3B. Dalton held this assumption so strongly that it prevented him from perceiving that the formula of water is H2O as shown by Gay-Lussac's data and Avogadro's reasoning. The rule insisted that because oxygen and hydrogen combine in just one way, the product should be HO (Mason, 1962). This assumption also led him to conclude that the mass ratio of hydrogen to oxygen was 1:7 when he could have arrived at 1:14 (difference between 14 and 16 being experimental error). It was not until Cannizaro cleverly reargued Avogadro's hypothesis at the Karlsruhe Congress in 1860 that oxygen and hydrogen were accepted as diatomic gases and the impasse between Dalton and Gay-Lussac was resolved in Gay-Lussac's favour. The rule was then refined to state that when two elements combine, the ratios are simple integers and that 1:1 has no natural precedence over 1:2, 2:3, 1:3, etc. Still, the rule of greatest simplicity was "Dalton's greatest single contribution to the formulation of an atomic theory" (Nash, 1957). The rule was a crucial theoretical advance because it insisted that chemical reactions are orderly and that predictable changes occur between discrete invisible particles.



Whereas the "rule of greatest simplicity" was a barrier to Avogadro's conclusion that equal gas volumes contain equal numbers of particles; it also was the key to the atomic theory's success at the start of the 19th Century. Dalton's belief that reacting elements comprised a multitude of identical and discrete particles was a significant improvement on Newton's and Boyle's corpuscular theories because Dalton's theory explained and was built on experimental data. The atomic theory was supportable even if the 'equal gas volumes contain different numbers of particles' was not. Despite its weaknesses, Dalton's belief in atoms was theoretically sound and allowed him to make the crucial predictions that suggested the decisive experiments that tested the theory's predictions and produced, over the next 50 years, the key tenets of the atomic theory. Still, the strength of Dalton's reputation, the rule of greatest simplicity and Dalton's "conception of a gas as solidly packed with particles, like a pile of shot" (Nash, 1957) inhibited Gay-Lussac's and Avogadro's theoretical advances.



Dalton's belief that gas particle size is directly related to its mass is not uncommon. Students predict that when water is electrolysed, the one volume of oxygen produced will occupy a greater volume than the two volumes of hydrogen at the same temperature and pressure because oxygen with 8 protons + 8 neutrons is much larger than hydrogen's single proton (Gabel, Samuel & Hunn, 1987). This alternative conception could be avoided by explicitly showing how this idea hindered the development of Avogadro's hypothesis [which "is not self-evident" (Gabel, 1999)]. The history of science is more than an interesting story because it can show the way to effective conceptual growth and conceptual change. Histories like this story also show students that science is a human enterprise, scientists do make mistakes, and the scientific academy is rigorous and imaginative. Incorporating history into the teaching of chemistry may well increase chemistry's appeal to students.



Atoms are Independent, Invisible and Indivisible



Dalton observed that when two elements react to form a new substance, the reacting masses always react in simple and consistent ratios. Gay-Lussac also recognised this pattern in the volumes of gas that reacted in similar reactions. As a result, Dalton and Gay-Lussac understood that when two elements react to form a specific compound, they always combine in the same simple proportions. How could this be explained in terms of the underlying structure of matter?



During the 1780s, Lavoisier's accurate experiments established the Law of Conservation of Matter by demonstrating that the total mass of the products always equaled the mass of reactants. Matter was neither created nor destroyed. In 1797 Proust showed that for each compound he studied, the reacting elements combine in a constant ratio yielding the Law of Constant Composition. Dalton's experiments with nitrogen and oxygen showed that three oxides were possible: nitrous oxide, nitric oxide and nitrogen dioxide. He first demonstrated that when the reacting conditions for each oxide were present, only that oxide resulted and his data obeyed the Law of Constant Composition. Three different combining ratios for nitrogen and oxygen led him to formulate the Law of Multiple Proportions. These laws are most remarkable when we remember that Proust, Lavoisier and Dalton worked with limited knowledge and equipment. But Dalton was an insightful theoretician and he saw the pattern that others missed. He saw what the Law of Constant Composition and the Law of Multiple Proportions were telling him: that the simple and constant ratios reported by Proust in France and Richter in Germany could only be explained if hydrogen, nitrogen, oxygen and the other known elements were made up of indivisible, invisible and independent particles that combine in simple and predictable ratios. This insight became the cornerstone of his atomic theory.



Nash (1957) calls Dalton the "skilful observer" who "contributed a notably plausible, precise and unambiguous statement of the basic postulates of the atomic theory" that was based on his conceptual scheme of how matter is constructed and behaves.



Proust and his contemporaries held the critical data in their hands and failed to see the significance of what they "knew". With the advent of Dalton's atomic theory, the new beliefs it encouraged brought about a remarkable sharpening of the empiricist's vision. They were told what to look for, and where and how to look for it - and behold, it was there. Dalton's ... fundamental contribution was the powerful stimulus to investigation provided by his conceptual scheme. (Nash, 1957)



But how did Dalton see what others "knew" yet failed to perceive? A striking feature of his own accounts of the atomic theory is the consistent way he uses "atom" to denote a fundamental elemental particle, one that is indivisible and too small, in his opinion, to ever be seen. He talks of compound atoms (our molecules) and develops his theory using "thought experiments". In an 1810 lecture to the Royal Institution using "the Newtonian doctrine of repulsive atoms and particles, I set to work to combine my atoms on paper". Dalton's thought experiment tells how, at length, he deduced that the atmosphere was a mixture, not a compound. In other accounts he explains how his thinking about the available data (quite limited data in scope and number) led him to the "rule of greatest simplicity" and, subsequently, to his atomic theory. Both Nash, and Toulmin and Goodfield reveal how important thought experiments were in directing Dalton away from the pursuit of unsystematic data towards the fruitful concepts of his atomic theory.



Popular textbook accounts of the scientific method represent science as a logical procession from observation through experiment to hypotheses culminating in a new or revised theory. Neither Boyle (Toulmin & Goodfield, 1962) nor Dalton followed this route; instead, 'intuitive' theories guided their thinking. "Dalton did not proceed in a clear-cut fashion from postulate to argument, ... rather, he followed the reverse course" (Nash, 1957). While thought experiments and theorizing before the crucial experiment is conducted is common in the history of the quantum theory, it is less often supposed to have occurred in the 18-19th Centuries. But this is a striking feature of the stories of Lavoisier, Dalton, Gay-Lussac and Avogadro. The fruitful 'intuitive' theory is a hallmark of their thinking; and their investigations were purposefully focused by the predictions that emerged from their theories. Theory was pre-eminent in their thinking and Chalmers (1997) shows that theory is an indispensable ingredient in all scientific progress; that is, no scientist can make sense of his or her data without an organizing framework. The theory may soon need to be modified, but such a theory is better than no theory at all. This principle should be pursued in secondary science teaching because it helps students understand that science is a way of knowing rather than a body of knowledge. Such thinking is the foundation of 'working scientifically" (or 'inquiry") and is the rationale for most modern science curricula (e.g., Queensland Schools Curriculum Council, 1999).



The thought experiment was prominent in Newton's corpuscular theory of light and in Boyle's method of modelling elastic gas molecules as tiny springs. The thought experiment is shown to be an excellent tool for doing science. The benefit for students in sharing these stories is the legitimation of imagination and creativity in science. Students should be encouraged to play with ideas as this will likely increase their interest in scientific thinking. But they are unlikely to understand the power of theorising about data and evidence without exposure to the historic struggles of scientists like Newton, Dalton and Avogadro. All of these scientists dealt with things they could not see, yet in their mind's eye they "saw" the important concepts because they used a theoretical lens to interpret their data.



The Kinetic Theory of Gases



From early on, Boyle was a supporter of [Newton's] corpuscular philosophy, believing that all the properties and changes of material things could eventually be explained by the shapes, motions and arrangements of their tiny constituent particles. (Toulmin & Goodfield, 1962)



Newton proposed that a gas's particles were evenly spread through its enclosing space due to the particles' short-range repulsive forces. Boyle attributed the even spacing of gases to their springiness and modelled gas particles as tiny coiled springs. Boyle's experiments led him to insist that gas particles must possess mass, characteristic shapes and motion. He also argued that the inverse volume-pressure relationship for gases could only be explained in atomistic terms. However, this was all pre-Dalton and Boyle's atomism was principally philosophical, that is, his theoretical explanation for the data he collected demanded that gas particles be as he described them. Still, his dynamic particles with no intervening matter other than Newton's universal aether, supports the modern picture. When the Michelson-Morley experiment (1887) dispelled the aether, the modern image of a gas composed of independent, invisible and immutable particles became credible.



Dalton held views contrary to Boyle that inhibited the formulation of the kinetic theory as we know it. Dalton visualized a gas as a "pile of shot". Dalton's gas particles were single atoms in contact with each other and each atom's size matched its mass. This view differed from the modern trend that atomic radii gradually increase with rising atomic number because he saw oxygen as many times larger than hydrogen. Gay-Lussac, on the other hand, insisted that "the distances between individual gaseous particles are assumed to be so great in comparison with their diameters that the variable attractive forces between neighboring particles are negligible" (Nash, 1957). As early as 1738, Bernoulli provided a modern explanation of gas pressure and volume by assuming that "the atoms of a gas were in random motion, the pressure of the gas being nothing more than impact of the atoms on the wall of the containing vessel" (Mason, 1962). Grasping the significance of these ideas, Avogadro proposed that equal volumes of different gases at the same temperature and pressure contain the same number of particles. In 1814, Ampère, drew a similar conclusion. Avogadro then argued in his famous paper of 1811 that oxygen and hydrogen must be diatomic to explain how 1 volume oxygen + 2 volumes hydrogen produce 2 volumes of water vapour.



But Dalton could not accept Gay-Lussac's data nor Avogadro's hypothesis. Dalton's "the rule of greatest simplicity" said that the ratio of hydrogen : oxygen in water must be 1:1 or HO. He also believed that a volume of oxygen comprised numerous atoms which, in this example, we will call n atoms of oxygen. When the n atoms in one volume of oxygen react with hydrogen, two volumes of water vapour result. Dalton's theory disallowed there being 2n molecules of water because only n oxygen atoms were available and oxygen atoms are indivisible. Thus, he proposed that each of the two volumes of water vapour contained n/2 molecules of OH. To justify the assertion that one volume of oxygen contains n atoms and one volume of water vapour contains n/2 molecules, he wrote, "the globular particles in a volume of pure elastic fluid [gas], ... must be analogous to that of a square pile of shot ... each particle rests of four particles below" (Dalton, 1808). His conception that gas particles are in contact with each other, led him to conclude that a two atoms per particle gas (water, HO) occupies twice the volume of a one atom per particle gas (oxygen, O).



Dalton's thinking is repeated time over in science and in the classroom. In 1833 Faraday "established that the same amount of electricity brought about the decomposition of the same number of equivalents of different chemical substances" (Mason, 1962) and went on to show that the same amount of electricity yielded the same quantity of, say, hydrogen or zinc, from all of their compounds. Maxwell was unable to accommodate these findings within his or Faraday's theories and wrote that



we leap over this difficulty by simply asserting the fact of the constant value of the molecular charge, and that we call this constant molecular charge .... one molecule of electricity.... It is extremely improbable however that when we come to understand the true nature of electrolysis we shall retain in any form the theory of molecular charges. (Mason, 1962)



Unbeknown to them, Faraday and Maxwell were adding support to the already strong atomic theory by asserting that particles and charge are quantised. The only theory that allows such a conclusion is the one that says that all matter (and now electric charge) is composed of discrete, predictably behaved, tiny particles called atoms (and charge). Faraday denied the atomic theory per se and conceived, like Dalton, "that matter is everywhere present, and there is no intervening space unoccupied by it" (Mason, 1962); Maxwell skeptically expected his conclusion to soon be denied and Dalton consistently rejected Avogadro's refinement of his own atomic theory. These examples amply demonstrate the conceptual compromises that even one alternative conception can wreak in a conceptual framework. Similarly, science education has shown the problems that a continuous view of matter and inductive projection of mass properties onto particles creates in the classroom (de Vos & Verdonk, 1996).



Macroscopic, Submicroscopic and Symbolic Representations



Gabel (1999) discusses Johnstone's (1991) observation that chemical phenomena are explainable in three ways - in macro, sub-micro and symbolic terms. Reactions are visible as changes in mass, state, volume, solubility, colour and temperature. But descriptive information has limited power in explaining what happens at the particle level, so chemists turn to submicroscopic and symbolic models. The macroscopic-submicroscopic-symbolic triangle that has proved so valuable in explaining chemical phenomena was first used by Dalton. Once Dalton saw what the Laws of Conservation of Mass, Constant Composition, and Multiple Proportions were telling him - that all matter is composed of invisible and indivisible atoms - he realized that he had to explain his observations in terms of particles and symbols or his theory would be neither credible nor communicable. He tells how he manipulated his "atoms" and "compound atoms" on paper to show that the atmosphere is a mixture, not a compound. And he invented a set of symbols to systematically describe the compounds and reactions he observed. Dalton's symbols were quickly replaced by Berzelius' one or two initials taken from each element's name; nevertheless, it was Dalton who saw the need to symbolize chemical action in an elegant and parsimonious way. It can therefore be argued that Dalton is the father of the tripartite macro, sub-micro and symbolic ways of describing and explaining chemical reactions.