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Chemistry Lec ch4 - Chapter 4 The Divisibility of Matter...

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Chapter 4
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The Divisibility of Matter Infinitely Divisible for any two points there is always a point between Ultimate Particle upon division eventually a particle is reached which can no longer be divided “Nothing exists except atoms and empty space; everything else is opinion.” - Democritus 460–370 B.C.
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Dalton’s Atomic Theory 1. Elements are composed of atoms tiny, hard, unbreakable, spheres 2. All atoms of an element are identical so atoms of different elements are different every carbon atom is identical to every other carbon atom they have the same chemical and physical properties but carbon atoms are different from sulfur atoms they have different chemical and physical properties John Dalton (1766-1844)
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Dalton’s Atomic Theory 1. Atoms combine in simple, whole-number ratios to form molecules of compounds because atoms are unbreakable, they must combine as whole atoms the nature of the atom determines the ratios it combines in each molecule of a compound contains the exact same types and numbers of atoms Law of Constant Composition Chemical Formulas
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Dalton’s Atomic Theory 1. In chemical reactions, atoms are not broken or changed into another type. all atoms present before the reaction are present after atoms are not created or destroyed, just rearranged therefore the total mass will remain the same Law of Conservation of Mass atoms of one element do not change into atoms of another element in a chemical reaction cannot turn Lead into Gold by a chemical reaction
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Sizes of Atoms using compositions of compounds and assumed formulas, Dalton was able to determine the relative masses of the atoms Dalton based his scale on H = 1 amu we now base it on C-12 = 12 amu exactly unit = atomic mass unit amu or dalton absolute sizes of atoms mass of H atom= 1.67 x 10 -24 g volume of H atom = 2.1 x 10 -25 cm 3
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Some Notes on Charge Two Kinds of Charge called + and – Opposite Charges Attract + attracted to – Like Charges Repel + repels + – repels – To be Neutral, something must have no charge or equal amounts of opposite charges
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Maintaining and Restoring Charge Balance
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Plum Pudding Atom
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Rutherford’s Experiment How can you prove something is empty? put something through it use large target atoms use very thin sheets of target so do not absorb “bullet” use very small particle as bullet with very high energy but not so small that electrons will effect it bullet = alpha particles, target atoms = gold foil α particles have a mass of 4 amu & charge of +2 c.u. gold has a mass of 197 amu & is very malleable
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Rutherford’s Experiment Lead Box Radioactive Sample Gold Foil Fluorescent Screen Alpha Particles Striking Screen
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Rutherford’s Results Over 98% of the α particles went straight through About 2% of the α particles went through but were deflected by large angles About 0.01% of the α particles bounced off the gold foil “...as if you fired a 15” canon shell at a piece of tissue paper and it came back and hit you.”
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