Chapter 2 Lecture Notes - Chapter 2 Atoms Molecules and...

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Atoms, Molecules, and Ions Chapter 2 Atoms, Molecules, and Ions Jim Geiger Cem 151
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Atoms, Molecules, and Ions Atomic Theory of Matter The theory of atoms: Original to the Greeks Leuccipus, Democritus and Lucretius (Aristotle thought they were nuts) John Dalton (1805-1808) Revived the idea and made it science by measuring the atomic weights of 21 elements. That’s the key thing because then you can see how elements combine.
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Atoms, Molecules, and Ions Dalton’s Postulates Each element is composed of extremely small particles called atoms. Tiny balls make up the world
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Atoms, Molecules, and Ions Dalton’s Postulates All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements. O N
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Atoms, Molecules, and Ions Dalton’s Postulates Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions. (As far as Dalton knew, they couldn’t be changed at all). O N O N Red O’s stay Os and blue N’s stay N’s.
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Atoms, Molecules, and Ions Dalton’s Postulates Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms. H N NH 3 ammonia Chemistry happens when the balls rearrange
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Atoms, Molecules, and Ions Law of Constant Composition Joseph Proust (1754–1826) Also known as the law of definite proportions. The elemental composition of a pure substance never varies. The relative amounts of each element in a compound doesn’t vary. H N NH 3 ammonia ammonia always has 3 H and 1 N.
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Atoms, Molecules, and Ions Law of Conservation of Mass The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place. 3H 2 + N 2 2N H 3 ammonia The atoms on the right all appear on the left
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Atoms, Molecules, and Ions The Electron Streams of negatively charged particles were found to emanate from cathode tubes. J. J. Thompson (1897). Maybe atoms weren’t completely indivisible after all.
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Atoms, Molecules, and Ions The Electron Thompson measured the charge/mass ratio of the electron to be 1.76 × 10 8 coulombs/g. How? by manipulating the magnetic and electrical fields and observing the change in the beam position on a fluorescent screen.
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Atoms, Molecules, and Ions Millikan Oil Drop Experiment measured charge of electron Univ. Chicago (1909). How? Vary the electric field (E) until the drops stop. Vary the charge (q) on the drop with more X-rays. Get a multiple of 1.6x10 -19 Coulombs. The charge of 1 electron. Eq = mg You set E, measure mass of drop (m) & know g. Find q.
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Atoms, Molecules, and Ions Radioactivity: The spontaneous emission of radiation by an atom.
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