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General Chemistry Online: Companion Notes: Atoms & ions: Dalton's atomic theory 3/29/15, 9:28 AM 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 Page 1 of 2 General Chemistry Online: Companion Notes: Atoms & ions: Dalton's atomic theory: Dalton's postulates 3/29/15, 9:29 AM Dalton's postulates Page 1 | 2 | 3 | 4 | 5 | 6 | Back | Next 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 http://antoine.frostburg.edu/chem/senese/101/atoms/print-dalton-postulates.shtml Page 1 of 2 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 Page 1 of 1