Dalton atomic weights

Content: composition, formulas, stoichiometry

Level: introductory

Reference: John Dalton, A New System of Chemical Philosophy, Part I, (Manchester, 1808, 1810)

Notes: John Dalton (1766-1844) is best known for formulating an atomic theory that proved to be enormously fruitful, even though it turned out to be incorrect in several important details. His theory pictured chemical compounds much as we do today, as atoms of different elements bound together. The laws of definite proportions and multiple proportions follow naturally from such an atomistic view of chemical combination. Dalton focused on properties of atoms that were amenable to empirical investigation, namely their relative masses. The fruitfulness of Dalton's theory can be seen in the research on combining masses and relative masses of the elements and their compounds that followed it. As valuable and fruitful as Dalton's work certainly was, it was mistaken in several details. Dalton believed that atoms of the same element were identical. He also seems to have believed that they were indestructible.

When Dalton set out his atomic hypothesis, he included a table of atomic weights relative to hydrogen = 1. The table was based on chemical analyses and on certain assumptions about what we would call molecular formulas. One assumption was that if two elements formed only one compound, that compound consisted of a single atom of each element (for example, water would be HO and ammonia NH). Some of Dalton's contemporaries saw these assumptions as arbitrary. This exercise asks whether the atomic weights Dalton assigned to hydrogen, nitrogen, and oxygen were consistent with his assumptions and with the chemical analyses available to him.

Further information: A detailed summary of key primary literature on multiple proportions, the atomic hypothesis, and atomic weights, including some quantitative treatment of data may be found in Leonard Nash, "The Atomic-Molecular Theory," in James Bryant Conant, ed., Harvard Case Histories in Experimental Science, vol. 1 (Cambridge, MA: Harvard, 1957), pp. 215-321.

Solutions: To download solutions, go to:

Copyright 2003 by Carmen Giunta. Permission is granted to reproduce for non-commercial educational purposes.

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