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Atomic Theory. John Dalton First to develop a theory on the structure of an atom 1800s Based on 5 major principles.

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Presentation on theme: "Atomic Theory. John Dalton First to develop a theory on the structure of an atom 1800s Based on 5 major principles."— Presentation transcript:

1 Atomic Theory

2 John Dalton First to develop a theory on the structure of an atom 1800s Based on 5 major principles

3 Daltons 5 Principles 1) All matter is made of indivisible atoms 2) All atoms of a given element are identical in their chemical and physical properties 3) Atoms of different elements have different chemical and physical properties Daltons notation of elements: 21 = water, 22 = ammonia,

4 Daltons 5 Principles 4) Atoms of different elements combine in simple, whole number ratios to form compounds Ex: 2 H + 1 O H 2 O 5) Atoms cannot be subdivided, created, or destroyed when they combine, separate or rearrange in reactions *Law of Conservation of Mass

5 Problems with Daltons ideas JJ Thomson was able to prove that atoms were made of smaller particles that contained charge Sent electricity through a cathode ray tube – gas inside glowed

6 J.J. Thomson Used a cathode ray tube to prove the small particles must have a NEGATIVE charge = Electron Electric beam bent with a magnet Cathode Ray Tube

7 J.J. Thomson Atoms were electrically neutral, so there must be a + particle to cancel out the – charge from the electron 1897 – developed the Plum Pudding model of an atom Positive ball containing electrons

8 Robert Millikan Oil-drop Experiment Determined the charge on an electron to be × coulomb Determined mass of electron to be 1/2000 of a proton

9 Ernest Rutherford Student of Thomson Conducted the Gold Foil Experiment in 1909 to disprove the Plum Pudding model

10 The Gold Foil Experiment Sent a beam of + charges (alpha particles) through a very thin piece of gold foil Angles of deflection were measured

11 Gold Foil - Conclusion Since most alpha (+) particles passed straight through, most of the foil must actually be empty regions – not a solid + sphere like Thomson believed Positive charge and most of the mass of an atom must be found in the core/center = NUCLEUS

12 Rutherfords Model of an Atom

13 Niels Bohr There are only certain discrete orbits allowed in an atom and no others Electrons must be in one of these orbits

14 Erwin Schroedinger 1926 Modified atomic structure to include electrons location in a cloud or shell instead of a predictable orbit

15 James Chadwick 1932 – discovered the neutron A neutron contained no charge and a mass equivalent to that of a proton


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