CH02 MSJ jlm - Chapter 2 Chapter Atoms and Elements Ernest...

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1 Atoms and Elements Atoms and Elements Chapter 2 Chapter 2 John Dalton 1766-1844. Conceived atomic weights. Ernest Rutherford 1871-1937.* Discoverer of atomic nucleus. D. I. Mendeleev 1834-1907. Periodic Table. Predicted elements.
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2 John Dalton: Elements are composed of atoms . All atoms of an element are identical (chemically) . (Dalton stressed “identical in weight” but he didn’t know about isotopes) In chemical reactions, the atoms are not changed. Compounds are formed when atoms of more than one element combine. The Atomic Theory of Matter The Atomic Theory of Matter (e.g., H 2 O, C 6 H 6 , C 12 H 22 O 11 but not H 2 , Cl 2 )
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3 For example, water (H 2 O) is always a ratio of two hydrogen atoms to one oxygen atom. Law of Constant Composition In a compound, relative amounts and kinds of atoms are fixed. Specifically, water is always 11.1% hydrogen and 88.9% oxygen (by weight ). For water, O/H = 88.9/11.1 = 8/1 ( by weight ) Dalton perceived that this constant ratio of mass of different elements in a compound reflected a specific ratio of atoms, with each element having its specific, but unique weight. But he was confused about the exact number of atoms in a compound. For example, he thought water had a formula of HO (and not H 2 O).
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4 The ancient Greeks were the first to postulate that matter consists of indivisible constituents. Later scientists realized that the atom consisted of charged (+ or -) entities. The Discovery of Atomic Structure The Discovery of Atomic Structure Cathode Rays and Electrons Cathode Rays and Electrons A cathode ray tube (CRT) is a hollow vessel with an electrode at either end. A high voltage is applied across the electrodes. A charged particle will have its path bend in either an electric or magnetic field.
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5 Cathode Rays and Electrons Cathode Rays and Electrons The Discovery of Atomic Structure The Discovery of Atomic Structure (electrons are charged (-) particles)
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6 The Discovery of Atomic Structure The Discovery of Atomic Structure a spot which is not affected by the electric field, Three spots are noted on the detector: a spot in the direction of the positive (+) plate, a spot in the direction of the negative (-) plate.
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7 β -radiation: Large deflection toward the positive plate corresponding to radiation which is negatively charged and of low mass. These β particles are light and of low mass. β
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This note was uploaded on 04/12/2011 for the course CHEM 1420 taught by Professor Acree during the Spring '08 term at North Texas.

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CH02 MSJ jlm - Chapter 2 Chapter Atoms and Elements Ernest...

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