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Early Atomic Theory
- 1. Early Atomic Theory If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms-little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied September 19, 2014 Bill Bryson’s Brief History of Nearly Everything,
- 2. Imaging the Atom • An atom cannot be ‘seen’ but it can be imaged using a Scanning Tunneling Microscope (STM) which ‘traces’ the outer electrons of atoms The red peaks are cesium atoms, on a background of gallium arsenide The STM can also move individual atoms around, giving rise to the field of nanotechnology. Guess what company invented the STM? (1981)
- 3. • STM worksby scanning an ultrasharp metal tip across a surface. When a voltage is applied between the tip and the surface, electrons tunnel between the two, the tunneling current being proportional to the distance between the tip and surface features. In this way molecular features on the surface can be mapped –Royal Society of Chemistry
- 4. • STM hasbeen improved with Atomic Force Microscopy (AFM), giving even higher resolutions The tip of an AFM microscope traces the structure of pentacene, producing the 1st image of a molecule (2009)
- 5. The Scientific Method • Science is a process which we use to gain a better understanding of reality • We have improved upon our model of the atom over time • These changes continue to happen because of the scientific method
- 6. How the ScientificMethod Works
- 7. Dalton and thebilliard ball • 1807 • Atoms are indivisible • Each element has its own type How the Scientific Method Works
- 8. New evidence: Crookesinvented the cathode ray tube, showing that (-) electrons exist
- 9. Thompson and theraisin bun • 1904 • Atoms are negative particles embedded in a positive sphere • The # of electrons determines element
- 10. New evidence: Rutherford’sgold foil experiment showed that atoms ‘scatter’ alpha particles http://www.kcvs.ca/site/projects/physics_files/rutherford/historical_scattering.html
- 11. Rutherford and thenucleus • 1911 • Geiger and Marsden shot alpha (+) particles at a piece of gold foil expecting them to go through • Some were deflected by something in the atom
- 12. • If Thompson’smodel was correct, the (+) particles should have gone right through • They didn’t, so Rutherford proposed that atoms have a dense, (+) nucleus
- 13. • Later, Chadwicktheorized that the nucleus also contains neutral neutrons (which also explained isotopes) - Dense nucleus containing protons and neutrons, surrounded by a cloud of tiny negative electrons
- 14. • Problems? –What do positive and negative do? – Also, as the electrons orbit, they would continuously lose energy
- 15. • If Rutherford’smodel was correct, the electrons would spiral into the nucleus and matter could not exist • Clearly, it does. How?
- 16. What else iswrong with our model? Remember Periodic trends: Ionization energy?: • Increases across a period and up a group. Right? Then why is B’s less than Be’s, O’s less than N’s…
- 17. Atomic Radii?: •Decreases across a period, Right? Then why do the transition metals within a period all have about the same size?
- 18. • Pg. 166 • Q. 3-8