Chem-Ch2 - C hapte 2 r Atoms and Elements Early Philosophy...

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Atoms and Elements Chapter 2
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2 Early Philosophy of Matter Some philosophers believed that matter had an ultimate, tiny, indivisible particle Leucippus and Democritus Other philosophers believed that matter was infinitely divisible Plato and Aristotle Since there was no experimental way of proving who was correct, the best debater was the person assumed correct, i.e., Aristotle
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3 Law of Conservation of Mass In a chemical reaction, matter is neither created nor destroyed Total mass of the materials you have before the reaction must equal the total mass of the materials you have at the end total mass of reactants = total mass of products Antoine Lavoisier 1743-1794
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5 Law of Definite Proportions All samples of a given compound, regardless of their source or how they were prepared, have the same proportions of their constituent elements Joseph Proust 1754-1826
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6 Proportions in Sodium Chloride 54 . 1 g 39.3 g 7 . 60 Na of mass Cl of mass = = a 100.0 g sample of sodium chloride contains 39.3 g of sodium and 60.7 g of chlorine a 200.0 g sample of sodium chloride contains 78.6 g of sodium and 121.4 g of chlorine 54 . 1 g 78.6 g 21.4 1 Na of mass Cl of mass = = a 58.44 g sample of sodium chloride contains 22.99 g of sodium and 35.44 g of chlorine 541 . 1 g 22.99 g 5.44 3 Na of mass Cl of mass = =
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7 Law of Multiple Proportions When two elements, (call them A and B), form two different compounds, the masses of B that combine with 1 g of A can be expressed as a ratio of small, whole numbers John Dalton 1766-1844
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Dalton’s Atomic Theory 1. Each element is composed of small indivisible particles called atoms . Atoms retain their identity during a chemical reaction. 2. All atoms of a given element are identical ; the atoms of different elements are different. 3. Atoms combine in fixed proportions to form molecules. 4. A chemical reaction involves only a rearrangement of atoms 5. Atoms of an element cannot be changed into atoms of another element.
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Modern Modifications Atoms are not indivisible, they contain subatomic particles. The existence of isotopes means that all atoms of an element are not exactly the same. These postulates were based on macroscopic observations. They are correct for the most part, but with a few exceptions.
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Atoms Very small 10 22 atoms in 1 penny Equivalent to 1 grain of sand in a sandbox the size of Texas Smallest characteristic of a given element Make up everything Different for each element
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12 Thomson’s Experiment The cathode rays are made of tiny particles These particles have a negative charge Every material tested contained these same particles These cathode ray particles became known as electrons
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A New Theory of the Atom Since the atom is no longer indivisible, Thomson must propose a new model of the atom to replace the first statement in Dalton’s Atomic Theory rest of Dalton’s theory still valid at this point Thomson proposes that instead of being a hard,
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This note was uploaded on 08/19/2011 for the course CHM 2045 taught by Professor Geiger during the Spring '08 term at University of Central Florida.

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Chem-Ch2 - C hapte 2 r Atoms and Elements Early Philosophy...

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