John Dalton, A New System of Chemical Philosophy, 1808)

	Vocabulary
	atom atomic number chemical change element isotope law of conservation of mass law of definite proportions law of multiple proportions mass stoichiometry

John Dalton (1766-1844) developed the first useful atomic theory of matter around 1803. In the course of his studies on meteorology, Dalton concluded that evaporated water exists in air as an independent gas. Solid bodies can't occupy the same space at the same time, but obviously water and air could. If the water and air were composed of discrete particles, Dalton reasoned, evaporation might be viewed as a mixing of water particles with air particles. He performed a series of experiments on mixtures of gases to determine what effect properties of the individual gases had on the properties of the mixture as a whole. While trying to explain the results of those experiments, Dalton developed the hypothesis that the sizes of the particles making up different gases must be different. He later wrote [1]

"...it became an object to determine the relative sizes and weights, together with the relative numbers of atoms entering into such combinations... Thus a train of investigation was laid for determining the number and weight of all chemical elementary particles which enter into any sort of combination one with another."

Dalton's atomic theory rests on the following postulates.

1. All matter consists of tiny particles. The existence of atoms was first suggested more that 2000 years before Dalton's birth. Atoms remained pure speculation through most of this time, although Newton used arguments based on atoms to explain the gas laws in 1687. (Newton's speculations about atoms in the Principia were carefully copied by hand into Dalton's notebooks.)

2. Atoms are indestructible and unchangeable. Atoms of an element cannot be created, destroyed, broken into smaller parts or transformed into atoms of another element. Dalton based this hypothesis on the law of conservation of mass and on centuries of experimental evidence.

   With the discovery of subatomic particles after Dalton's time, it became apparent that atoms could be broken into smaller parts. The discovery of nuclear processes showed that it was even possible to transform atoms from one element into atoms of another. But we don't consider processes that affect the nucleus to be chemical processes. The postulate is still useful in explaining the law of conservation of mass in chemistry. A slightly more restrictive wording is "Atoms cannot be created, destroyed, or transformed into other atoms in a chemical change".

3. Elements are characterized by the mass of their atoms. All atoms of the same element have identical weights, Dalton asserted. Atoms of different elements have different weights.

   With the discovery of isotopes, however, the postulate was amended to read, "Elements are characterized by their atomic number".

4. When elements react, their atoms combine in simple, whole-number ratios. This postulate suggested a practical strategy for determining relative atomic weights from elemental percentages in compounds. Experimental atomic weights could then be used to explain the fixed mass percentages of elements in all compounds of those elements!

   This effectively explained both the law of definite proportions and the law of multiple proportions.

Some of the details of Dalton's original atomic theory are now known to be incorrect. But the core concepts of the theory (that chemical reactions can be explained by the union and separation of atoms, and that these atoms have characteristic properties) are foundations of modern physical science.

	Dalton's Atomic Theory
	Page 1: Introduction Page 2: Take a quiz on Dalton's Atomic Theory

References

1. John Dalton and the Atomic Theory: The Biography of a Natural Philosopher, Elizabeth C. Patterson, Doubleday, New York, 1970, chapter VI. The quote was taken from notes for a lecture Dalton delivered in 1810 before the Royal Institution in London.
2. A Short History of Chemistry, J. R. Partington, Macmillan, London (1937).

Related Pages

FAQ: History of Chemistry: Dalton Timeline (Fred Senese, Frostburg State University)

What year should I cite for the origin of Dalton's atomic theory? http://antoine.frostburg.edu/chem/senese/101/history/faq/dalton-timeline.shtml (10/03/99)

John Dalton (Carmen Giunta, LeMoyne College)

Excerpts from Dalton's A New System of Chemical Philosophy published in 1808. Includes Dalton's table of atomic weights and and scanned atomic symbols. http://webserver.lemoyne.edu/faculty/giunta/dalton.html (10/17/98)

My Brother, John (Judy Moody, Bette Bridges)

This narrative biography of John Dalton is told from his brother's point of view. Contributed to the 1992 Woodrow Wilson Summer Institute on the history of chemistry. Includes a bibliography and a Dalton charicature. http://www.woodrow.org/teachers/ci/1992/Dalton.html (3/27/99)

Copyright © 1997-1999 by Fred Senese
Comments & questions to senese@antoine.frostburg.edu
Last Revised 12/10/01.
URL: http://antoine.frostburg.edu/chem/senese/101/atoms/dalton.shtml