02_250S05lec2_atomic_str_2008

02_250S05lec2_atomic_str_2008 - Atomic Structure and...

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Atomic Structure Atomic Structure and Bonding and Bonding
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Problems to do: Ch 2: 1,2,3,8,9,17 Plus: Do problem 2.14 First Lab Meeting: TODAY
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1 nm 10 nm 100 nm 1 μμ 10 μμ 100 μμ 1μμ 10μμ 100μμ 10μ Παρτιχλεσ ινΧρψσταλσ (2200Γραινσ220029 ανδ Πρεχιπιτατεσ Αχχελερατορ Μιχροσχοπε(ΣΤΜ,ΑΦΜ29 Μιχροσχοπε(ΤΕΜ29 Μιχροσχοπε(ΣΕΜ29
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The surface of an alloy of platinum and rhenium (PtRh). The light spots are Rh atoms, grey spots are Pt. The black spots are carbon impurities. Pt and Rh tend to cluster in small groups of the same species. The magnification you see on the screen is over 300 million .
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Rutherford’s Scattering Experiment alpha source ZnS Screen: Backscattered flashes Fuzzy main spot (What Thompson would have expected) gold foil particle beam lead box ZnS Screen Faint flashes off-axis
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Rutherford Concludes in 1913: 1) Scattering of the alphas must be due to encounters with charged atomic particles that are much smaller than the atom . 2) There must be some positive electric charges inside the atom to compensate for negative charges of the electrons. 3) There must be stuff in the atom that is much heavier than an electron 4) Atoms must be mostly empty space . 5) Thus, an atom has a small core (the nucleus ) which contains most of the mass and carries a positive charge that attracts the negative electrons and keeps them in orbit.
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Atoms are subdivided into more fundamental units: Electron identified in 1897 by Thomson: Carries unit of negative electrical charge Mass is about ~1/2000 th that of proton. Proton identified in 1919 by Rutherford Principal constituent of nucleus Carries unit of positive charge. Mass is 1836 times that of electron Neutron is later identified in 1932 by Chadwick: Another primary particle in nucleus Carries no net electrical charge. Mass is approximately that of proton
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BUT : If the atom is like this little solar system, with electrons spinning around the protons (where the centripetal forces on the electron presumably balance the electrostatic attraction to the protons), classical theory definitely states that the electron should RADIATE ENERGY RADIATE ENERGY and therefore slow down and crash into the nucleus! When this problem is solved, things will never be the same . ..
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Niels Bohr 1913: Theory for orbits of electrons in atoms: 1. Electrons have certain allowed states in which they can move without radiating . 2. The allowed states have well-defined
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02_250S05lec2_atomic_str_2008 - Atomic Structure and...

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