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Foundations of Dalton's atomic theory

Vocabulary
atom*
atomic weight*
chemical change*
element
hypothesis*
isotope*
law of conservation of mass*
law of definite proportions*
law of multiple proportions*
nucleus*
weight*

Dalton's atomic theory makes the following assumptions:

  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".

    Dalton's Notebook
    A page from Dalton's notebook, burned
    in the bombing of Manchester
    in World War II. Note the
    incorrect formula for water.

  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. (Dalton used the word "weight" rather than mass, and chemists have called atomic masses "atomic weights" ever since).

    We now know that atoms of the same element sometimes have slightly different masses, but always have identical nuclear charge. In modern atomic theory, the postulate has been amended to read: "Elements are characterized by the nuclear charge of their atoms".

  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!

    By suggesting that compounds contained characteristic atom-to-atom ratios, Dalton effectively explained the law of definite proportions.

  5. When elements react, their atoms sometimes combine in more than one simple, whole-number ratio. Dalton used this postulate to explain why the weight ratios of nitrogen to oxygen in various nitrogen oxides were themselves simple multiples of each other. Even Dalton's critics were impressed by the power and simplicity of his explanation, and it persuaded many of them that his atomic theory was worthy of further investigation.

Unfortunately, Dalton included an additional postulate that prevented his theory from being accepted for many years. When atoms combine in only one ratio, Dalton said, "..it must be presumed to be a binary one, unless some cause appear to the contrary" [2]. He had no experimental evidence to support this postulate, and it lead him to mistakenly assume that the formula of water was OH and the formula of ammonia was NH. As a result, Dalton's atomic weights for oxygen and nitrogen were incorrect and his experimental data did not support many of the conclusions he drew from it.

A consistent set of atomic weights was absolutely essential before the theory could be accepted and applied. Next, we'll see how Dalton's postulates can be used to estimate atomic weights from experimental data, and how they explain three basic laws of chemistry.

Dalton's Atomic Theory
Page 1: Introduction
Page 2: Dalton's assumptions Five steps forward and one step back.
Page 3: Atoms in compounds An "invisible hand" fixes atom ratios in compounds
Page 4: Atoms in reactions Dance of the atoms
Page 5: Learning check Take a quiz on Dalton's Atomic Theory
Page 6: References and resources

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General Chemistry Online! Dalton's postulates

Copyright © 1997-2005 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 07/25/05.URL: http://antoine.frostburg.edu/chem/senese/101/atoms/dalton-postulates.shtml