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Atomic theory

Arguably the most important of all Dalton's investigations are concerned with the atomic theory in chemistry. While his name is inseparably associated with this theory, the origin of Dalton's atomic theory is not fully understood. The theory may have been suggested to him either by researches on ethylene (olefiant gas) and methane (carburetted hydrogen) or by analysis of nitrous oxide (protoxide of azote) and nitrogen dioxide (deutoxide of azote), both views resting on the authority of Thomas Thomson.

From 1814 to 1819, Irish chemist William Higgins claimed that Dalton had plagiarised his ideas, but Higgins' theory did not address relative atomic mass. Recent evidence suggests that Dalton's development of thought may have been influenced by the ideas of another Irish chemist Bryan Higgins, who was William's uncle. Bryan believed that an atom was a heavy central particle surrounded by an atmosphere of caloric, the supposed substance of heat at the time. The size of the atom was determined by the diameter of the caloric atmosphere. Based on the evidence, Dalton was aware of Bryan's theory and adopted very similar ideas and language, but he never acknowledged Bryan's anticipation of his caloric model. However, the essential novelty of Dalton's atomic theory is that he provided a method of calculating relative atomic weights for the chemical elements, which provides the means for the assignment of molecular formulas for all chemical substances. Neither Bryan nor William Higgins did this, and Dalton's priority for that crucial innovation is uncontested.

A study of Dalton's laboratory notebooks, discovered in the rooms of the Manchester Literary and Philosophical Society, concluded that so far from Dalton being led by his search for an explanation of the law of multiple proportions to the idea that chemical combination consists in the interaction of atoms of definite and characteristic weight, the idea of atoms arose in his mind as a purely physical concept, forced on him by study of the physical properties of the atmosphere and other gases. The first published indications of this idea are to be found at the end of his paper "On the Absorption of Gases by Water and other Liquids" already mentioned. There he says:

Why does not water admit its bulk of every kind of gas alike? This question I have duly considered, and though I am not able to satisfy myself completely I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases.

He then proposes relative weights for the atoms of a few elements, without going into further detail. However, a recent study of Dalton's laboratory notebook entries concludes he developed the chemical atomic theory in 1803 to reconcile Henry Cavendish ’s and Antoine Lavoisier ’s analytical data on the composition of nitric acid, not to explain the solubility of gases in water.

The main points of Dalton's atomic theory, as it eventually developed, are:

In his first extended published discussion of the atomic theory (1808), Dalton proposed an additional (and controversial) "rule of greatest simplicity". This rule could not be independently confirmed, but some such assumption was necessary in order to propose formulas for a few simple molecules, upon which the calculation of atomic weights depended. This rule dictated that if the atoms of two different elements were known to form only a single compound, like hydrogen and oxygen forming water or hydrogen and nitrogen forming ammonia, the molecules of that compound shall be assumed to consist of one atom of each element. For elements that combined in multiple ratios, such as the then-known two oxides of carbon or the three oxides of nitrogen, their combinations were assumed to be the simplest ones possible. For example, if two such combinations are known, one must consist of an atom of each element, and the other must consist of one atom of one element and two atoms of the other.

This was merely an assumption, derived from faith in the simplicity of nature. No evidence was then available to scientists to deduce how many atoms of each element combine to form molecules. But this or some other such rule was absolutely necessary to any incipient theory, since one needed an assumed molecular formula in order to calculate relative atomic weights. Dalton's "rule of greatest simplicity" caused him to assume that the formula for water was OH and ammonia was NH, quite different from our modern understanding (H O, NH). On the other hand, his simplicity rule led him to propose the correct modern formulas for the two oxides of carbon (CO and CO). Despite the uncertainty at the heart of Dalton's atomic theory, the principles of the theory survived.