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General Chemistry Online: Companion Notes: Atoms & ions: Dalton's atomic theory
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Dalton's atomic theory
We might as well attempt to introduce a new planet into the solar system, or to
annihilate one already in existence, as to create or destroy a particle of
hydrogen.
John Dalton, A New System of Chemical Philosophy, 1808)
John Dalton (1766-1844) developed the first useful atomic theory of matter around 1803.
Vocabulary
In the course of his studies on meteorology, Dalton concluded that evaporated water
exists in air as an independent gas. He wondered how water and air could occupy the
atom
same space at the same time, when obviously solid bodies can't. If the water and air were
chemical change
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
hypothesis
mixtures of gases to determine what effect properties of the individual gases had on the
stoichiometry
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 exceptional gift for recognizing and interpreting patterns in experimental data lead him from a
problem in meteorology to the idea of atoms as fundamental constituents of matter. He realized the
vital theoretical connection between atomic weights and weight relations in chemical reactions. He was
the first to associate the ancient idea of atoms with stoichiometry.
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: 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
General Chemistry Online! Dalton's atomic theory
Copyright © 1997-2005 by Fred Senese
Comments & questions to [email protected]
http://antoine.frostburg.edu/chem/senese/101/atoms/print-dalton.shtml
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General Chemistry Online: Companion Notes: Atoms & ions: Dalton's atomic theory: Dalton's postulates
3/29/15, 9:29 AM
Dalton's postulates
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Foundations of Dalton's atomic theory
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.)
Vocabulary
atom
atomic weight
chemical change
element
hypothesis
isotope
law of conservation of mass
law of definite proportions
law of multiple proportions
nucleus
2. Atoms are indestructible and unchangeable.
weight
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. (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
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General Chemistry Online: Companion Notes: Atoms & ions: Dalton's atomic theory: Dalton's postulates
3/29/15, 9:29 AM
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
A page from Dalton's notebook, burned
weight ratios of nitrogen to oxygen in various nitrogen in the bombing of Manchester
in World War II. Note the
oxides were themselves simple multiples of each
incorrect formula for water.
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
General Chemistry Online! Dalton's postulates
Copyright © 1997-2005 by Fred Senese
Comments & questions to [email protected]
Last Revised 07/25/05.URL: http://antoine.frostburg.edu/chem/senese/101/atoms/print-dalton-postulates.shtml
http://antoine.frostburg.edu/chem/senese/101/atoms/print-dalton-postulates.shtml
Page 2 of 2
General Chemistry Online: Companion Notes: Atoms & ions: Dalton's atomic theory: Atoms in compounds
3/29/15, 9:29 AM
Atoms in compounds
Dalton's atomic theory: Page 1 2 3 4 5 6 >>
The law of fixed composition
1 atom C @ 12 u each = 12 u C
2 atoms O @ 16 u each = 32 u O
}
12/32
4 atoms C @ 12 u each = 48 u C
8 atoms O @ 16 u each = 128 u O
}
48/128 = 0.375 u C / u O
8 atoms C @ 12 u each = 96 u C
16 atoms O @ 16 u each = 256 u O
}
96/256 = 0.375 u C / u O
= 0.375 u C / u O
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
G en er
eraal C h em
emi ssttrryy O nnlli n e!
e! A
Attoms
oms in
in ccomp
ompou
oun d s
Copyright © 1997-2005 by Fred Senese
Comments & questions to [email protected]
Last Revised 07/25/05.URL: http://antoine.frostburg.edu/chem/senese/101/atoms/print-daltoncompounds.shtml
http://antoine.frostburg.edu/chem/senese/101/atoms/print-dalton-compounds.shtml
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