law of conservation of mass
law of definite proportions
law of multiple proportions
Dalton's atomic theory makes the following assumptions:
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".
|A page from Dalton's notebook, burned|
in the bombing of Manchester
in World War II. Note the
incorrect formula for water.
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".
By suggesting that compounds contained characteristic atom-to-atom ratios, Dalton effectively explained the law of definite proportions.
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" . 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
Copyright © 1997-2005 by Fred Senese
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Last Revised 07/25/05.URL: http://antoine.frostburg.edu/chem/senese/101/atoms/print-dalton-postulates.shtml