Permission is granted to reproduce for non-commercial educational purposes. 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.įurther 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. An atom is the smallest unit of an element that can participate in a chemical. Matter is composed of exceedingly small, indivisible particles called atoms. Here are the postulates of Dalton’s atomic theory. Some of Dalton's contemporaries saw these assumptions as arbitrary. First published in 1807, many of Dalton’s hypotheses about the microscopic features of matter are still valid in modern atomic theory. 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). The table was based on chemical analyses and on certain assumptions about what we would call molecular formulas. When Dalton set out his atomic hypothesis, he included a table of atomic weights relative to hydrogen = 1. He also seems to have believed that they were indestructible. Dalton believed that atoms of the same element were identical. As valuable and fruitful as Dalton's work certainly was, it was mistaken in several details. 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. Dalton focused on properties of atoms that were amenable to empirical investigation, namely their relative masses.
The laws of definite proportions and multiple proportions follow naturally from such an atomistic view of chemical combination. His theory pictured chemical compounds much as we do today, as atoms of different elements bound together. 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. Reference: John Dalton, A New System of Chemical Philosophy, Part I, (Manchester, 1808, 1810)
Dalton atomic weights: teaching notes Dalton atomic weights Content: composition, formulas, stoichiometry