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Chem 177 Ch.2 Outline

Chem 177 Ch.2 Outline - Chem 177 Chapter 2 Atoms Molecules...

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Chem 177 Chapter 2: Atoms, Molecules, and Ions Page 1 2.1 Atomic Theory of Matter. Democritus (460 – 370 BC): proposed atoms (indivisible) Plato/Aristotle – no ultimately indivisible particle. 17 th century: atom concept re - invoked to explain gases. no chemistry Experiments – explored reactivities. Dalton – 1803 – 1807 Atomic theory to explain experiments - postulates Theories = models: gives explanation of events, not provable, but allow predictions. 1. Each element composed of small particles – atoms. 2. Atoms of one type have identical properties; atoms of different elements are different. 3. Atoms of an element – unchanged during chemical reaction. Atoms can neither be created nor destroyed. 4. Compounds formed from atoms are combined in fixed ratios. Explained some experimentally derived Scientific Laws. Scientific laws: summarize facts, no explanations. Law of constant composition (1.2) from postulate 4. Law of conservation of matter: in a chemical reaction, mass of the products is the same as the mass of the reactants – postulate 3. Dalton deduced Law of multiple proportions – atoms can combine in different ratios to form different compounds. i.e. H 2 O and H 2 O 2 2.2 The Discovery of Atomic Structure Dalton: theory based on chemical reactions, no direct evidence of existence of atoms. Subatomic particles make up atoms - charged and + charged particles like charges repel, unlike charges attract. electrons – cathode ray tubes – originate from the negatively charged electrode or cathode.
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Chem 177 Chapter 2: Atoms, Molecules, and Ions Page 2 J.J. Thomson – discovery of electron. All cathode ray tubes produced rays with the same properties. Negatively charged particles with mass. Determined the charge to mass ratio of electrons: 1.76 x 10 8 C/g. Robert Milliken (1909) measured the charge of an electron via the “oil drop experiment” Radiation was used to ionize oil droplets. The rate at which the droplets fell in an electric field was measured. Used to calculate the integral charge on droplets: 1.6 x 10 - 19 C. Using Thomson’s charge to mass ratio could calculate the mass of an electron J.J. Thomson – plum pudding model of atom: electrons embedded in positive charged sphere (like chocolate chips in a cookie). Rutherford (early 1900’s) radioactivity – alpha particles (high energy He nuclei) targeted gold foil Found that most particles passed through the foil but some “reflected 180 o Surmised: most of mass of atom concentrated in a very small volume, most of volume of matter (and atoms) empty space. Quote from Rutherford recalling the results of his gold foil experiment. "I had observed the scattering of alpha-particles, and Dr. Geiger in my laboratory had examined it in detail. He found, in thin pieces of heavy metal, that the scattering was usually small, of the order of one degree. One day Geiger came to me and said, "Don't you think that young Marsden, whom I am training in radioactive methods, ought to begin a small research?" Now I had thought that, too, so I said, " Why not let him see if any alpha-particles can be scattered through a large angle?" I may
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Chem 177 Ch.2 Outline - Chem 177 Chapter 2 Atoms Molecules...

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