Atomic History Dalton’S Atomic Theory – 1803

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Atomic History Dalton’s Atomic Theory – 1803 • All elements are composed of indivisible units called atoms. • All atoms of the same element are identical. • All atoms of different elements are different. • Compounds are formed by joining atoms of 2 or more elements in a definite whole number ratio • Changes made to Dalton’s theory in the modern theory – Atoms are divisible (made of smaller particles) – Isotopes of elements are the same chemically, but have different mass numbers • Isotopes have the same protons and electrons but different neutrons Discovery of Electrons • William Crooke – invented Crookes tube or Cathode Ray tube. – used electrically charged metal cathodes to send charged gas molecules across a vacuum tube. Gases would emit light at the opposites end of the tube. Believed the emission was cathode rays. Discovery of Electrons • JJ Thomson – used Crookes tube to show that cathode rays were actually negatively charged particles – used a wide variety of metals and gases and determined that all atoms contained these negatively charged particles – discovery of electrons led Thomson to form “plum- pudding model” • “Plum Pudding Model” – Equal, even distribution of protons and electrons • Rutherford’s Model (Lord Rutherford) – a small, dense, positively charged nucleus surrounded by seemingly empty space with negatively charged electrons orbiting the nucleus. – “Gold Foil Experiment” • Projected + Alpha particles at a thin gold foil surrounded by a ZnS screen that sparked when struck by Alpha particles • Most of the alpha particles passed straight through the foil, some were slightly deflected, while only a few were deflected by 90° or more • Results indicated that the positive charge was small and dense and the electrons were spread out in empty space. • *Rutherford could not explain how the electrons could orbit the nucleus without losing energy • Bohr Model (Niels Bohr 1913) – electrons are found in specific energy levels in which they can travel w/out radiating energy. – The lowest energy electrons are found closest to the nucleus. The further from the nucleus an electron is, the more energy it has. – When all electrons of an atom are in the lowest available energy levels the atom is in the ground state and is stable. • Charge – Cloud Model – Electrons are actually clouds of negative charge that are moving at speeds close to the speed of light. – The model shows the most probable location of an electron .

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This article was downloaded by: [Massachusetts Institute of Technology, MIT Libraries] On: 24 March 2011 Access details: Access Details: [subscription number 922844579] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37- 41 Mortimer Street, London W1T 3JH, UK Philosophical Magazine Series 5 Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t910588686 XL. Cathode Rays J. J. Thomson To cite this Article Thomson, J. J.(1897) 'XL. Cathode Rays', Philosophical Magazine Series 5, 44: 269, 293 — 316 To link to this Article: DOI: 10.1080/14786449708621070 URL: http://dx.doi.org/10.1080/14786449708621070 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. THE LONDON, EDINBURGH, AND DUBLIN PHILOSOPHICAL MAGAZINE AN]) JOURNAL OF SCIENCE. [FIFTR SERIES.] OCTOBER 1897.
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