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Atomic Models

Atomic Models. Atomic Theories. Atomic Theory – A Short History Fifth Century, BCE Democritus Believed matter was composed of very small, individual particles that were indestructible He called them “atomos” (meaning uncuttable)

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Atomic Models

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  1. Atomic Models

  2. Atomic Theories Atomic Theory – A Short History • Fifth Century, BCE • Democritus • Believed matter was composed of very small, individual particles that were indestructible • He called them “atomos” (meaning uncuttable) • His ideas persisted for centuries even though there was no experimental proof

  3. Atomic Theories JOHN DALTON - 1808 • Revised early Greek ideas into testable scientific theory • Based his Atomic Theory on three important concepts: 1. Law of Conservation of Mass 2. Law of Multiple Proportions 3. Law of Definite Proportions

  4. Atomic Theories Law of Conservation of Mass States that mass cannot be created or destroyed • the mass of the reactants equals the mass of the products

  5. Atomic Theories Dalton’s Principles • All matter is composed of extremely small particles called atoms which cannot be subdivided, created or destroyed • Atoms of a given element are identical in their physical and chemical properties Example: all water molecules freeze at 0 deg C and react with explosively with sodium

  6. Atomic Theories Dalton’s Principles (continued) 3. All atoms of one element are different from those of any other element 4. Atoms combine in simple, whole-numbered ratios to form compounds • Based on the Laws of Definite and Multiple Proportions

  7. Atomic Theories Dalton’s Principles (continued) 5. In chemical reactions, atoms are combined, separated or rearranged but NEVER created, destroyed or changed • Based on The Law of Conservation of Mass

  8. Atomic Theories Dalton, however, did all this work in the early 1800’s without ever knowing about subatomic particles! (Protons, Neutrons, Electrons)

  9. Atomic Models The Adventures of J.J. Thomson, Plum Pudding and the Electron! MMMMM…..That plum pudding looks delicious!

  10. I LOVE plum pudding! Atomic Models JJ Thomson: • English Physicist • Experimented with “cathode rays” • Was able to determine that the mass of the particles in the ray was much smaller than the mass of a hydrogen atom • The particle must be a smaller than an atom!

  11. Atomic Models JJ Thomson’s Cathode Ray Tube (CRT) Anode – attached to the positive terminal of the voltage source Cathode – attached to the Negative end of the voltage source

  12. Atomic Models A magnet placed near the tube deflected the beam, proving it was negatively charged. A small paddlewheel in the tube turned when hit by the beam, meaning the particles had mass.

  13. Atomic Models Thompson called the negatively charged particles in the beam “corpuscles” This name was later changed to “electrons” by one of Thompson’s associates.

  14. Atomic Models Thompson’s Conclusion: • An atom is a sphere of positive charge with electrons embedded in it “Plum-Pudding” Model of the Atom

  15. Atomic Models In 1909, one of his students, Ernest Rutherford, disproved the “Plum Pudding” model by doing is famous “Gold Foil” experiment.

  16. Atomic Models Rutherford’s experiment http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

  17. Atomic Models • What conclusions were drawn from Rutherford’s Experiment?

  18. Atomic Models Rutherford’s Model of the Atom: • Electrons orbit the nucleus just as planets • orbit the sun • Did not explain why the negatively charged electrons did not crash into the positively charged nucleus.

  19. Atomic Models Two years later, Danish physicist, Niels Bohr, proposed the Bohr Model of the atom

  20. Atomic Models Bohr’s Model • Electrons are located certain distances from the nucleus • Each distance is a certain quantity of energy that the electron can have

  21. Atomic Models • Electrons closest to the nucleus have the lowest energy, while the ones further away are in higher energy levels

  22. Atomic Models • The difference between two energy levels is called a quantum of energy. • Electrons can be only in an energy level, NOT between levels.

  23. Atomic Models Electrons and Light (pg. 92) Basic Information: • Light travels in waves • Each wave has a certain wavelength (distance between two consecutive peaks of a wave)

  24. Atomic Models Each wavelength has a certain frequency (the # of waves that pass through a specific point in one second)

  25. Atomic Model Light and the Visible Electromagnetic Spectrum • We can only see a small amount of the electromagnetic spectrum • Bohr found that if you pass a high voltage through a gas and look at it through a prism, it will have a distinctive pattern of colored lines called “line-emission spectrum”

  26. Atomic Models Bohr calculated that the line-emission spectrum corresponded to energy levels in the atom (which give off a different wavelength of visible light) • The colors are caused when an electron falls from its excited state to its ground state and gives off energy in the form of light! (pg. 94)

  27. Atomic Models Great video on line-emission spectrum http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/linesp16.swf

  28. Spectrum of a Fluorescent Light

  29. Spectrum

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