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Models of the Atom

Models of the Atom. Models of the Atom. 1907 Plum Pudding Model - Thomson. Rutherford Model 1911. Ernest Rutherford “atoms contain a very small heavy central positive nucleus, with the e- orbiting randomly around. Alpha a particles are He nuclei 2p + , and 2n o. 2 elementary charges.

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Models of the Atom

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  1. Models of the Atom

  2. Models of the Atom

  3. 1907 Plum Pudding Model - Thomson

  4. Rutherford Model 1911 Ernest Rutherford “atoms contain a very small heavy central positive nucleus, with the e- orbiting randomly around.

  5. Alpha a particles are He nuclei 2p+, and 2no. 2 elementary charges.

  6. Most a particles went straight through, but the ones that passed closest the Au nucleus were progressively more deflected.

  7. Gold foil experiment : atom is mostly empty space with dense positively charged nucleus.Neg e- move in circular orbits about the +nucleus.e- attracted to nucleusby electrostatic F

  8. What kept the neg e- from fall into the nucleus? -inertia from circular velocity of e- (angular momentum) balanced the electrostatic attraction of the nucleus. +

  9. Problems: • James Maxwell had proved earlier that accelerated charges radiate EM energy. • Since e- is in circular motion it is accelerated. • e- should lose E & spiral into the nucleus. • That does not happen! • Also - How did positive nucleus stay together?

  10. One interesting discovery of Rutherford’s experiment was he could estimate the diameter of the nucleus. He was able to use the repulsion of the alpha particle & the angle of deviation to estimate the diameter of the gold nucleus.

  11. The a particle repelled straight back would have to come to rest for a moment. At that moment its KE would be balanced by electrical PE.

  12. Angle q of deviation from undeflected path. Rutherford used scattering angles from many particles to make his measurement. q

  13. KE = Ep elc.KE = kQq/r • Q = charge on nucleus • q = charge on alpha particle • r is the “distance of closest approach” see table p 8 topic 9 V = kq/r.

  14. Ex 1: An a particle with KE = 7.7 MeV aimed at a gold nucleus is repelled straight back. Find the distance of closest approach. • 3 x 10-14 m.

  15. IB Questions Rutherford.

  16. Bohr looked at • Spectral Colors of gasses

  17. Bohr Model • http://www.youtube.com/watch?v=-YYBCNQnYNM&feature=player_detailpage#t=101s

  18. Continuous spectrumFrom sunlight

  19. Electric E supplied to gas tubes causes gases to emit light.

  20. Emission SpectrumWhen viewed through a prism or spectroscope, we see only certain l of light are emitted by each element. Bright Line Spectra

  21. Absorption Spectrumlight passed through cool gasses, absorption/dark line spectrum forms. Each gas absorbs certain l’s of light. Viewed through a prism, the same l’s that are emitted by each element, are absorbed by each element.

  22. Frequencies emitted exactly match the frequencies absorbed.

  23. Bohr States: the emitted & absorbed l correspond to energy emitted & absorbed by e- moving between orbits in the atom.Each gas has its own unique set of energies that can be absorbed & emitted. E = hf.

  24. When atom absorbs photon energy e- “jump” to higher E outer orbits. Atom is “excited”.When atom emits photons, e- “fall” to lower inner E orbits.No in between orbits possible, photons absorbed/emitted comes in discrete E amounts.

  25. Since electrons can only occupy certain orbits, the orbits themselves are quantized! To “jump” to a higher orbit, an e- absorbs an exact amount of energy equivalent to the difference between the E of the two orbits. The groundstate occurs when the e- is in the smallest radius orbit allowed.

  26. Light is produced during e- transitions. • It is not continuous but quantized in packets – photons. • A beam of light is made of trillions of photons produced from e- transitions. • More photons = brighter light. • Think of higher f photon as more massive – higher momentum.

  27. The frequencies/colors of the observed spectral lines correspond to the exact energies that e- absorb or emit as they move between allowed orbits in the atom.

  28. Diagrams

  29. Orbital Energy Levels/ Ionization Energy Each orbit is associated with a specific energy which corresponds to the minimum energy needed to totally strip an e- from that orbit. This ionizationenergy is more than the energy needed to jump between orbits. If an atom absorbs E equal to the orbit E it becomes ionized (charged). Orbits are named by quantum number/letter.

  30. Ionization Energy: e- stripped from atom if photon with sufficient energy absorbed.

  31. Ex 2: How much energy would be needed to ionize an electron:In the n=1 level of of Hydrogen?in the n = b or level of Mercury?In the n = 2 level of Hydrogen?

  32. Atoms must also absorb energy for the e- to jump to higher orbits.

  33. The amount of energy needed to jump up must exactly equal the E difference btw orbits. Ephoton = Ei - EfUse Ephoton = hf of the radiation.to find frequency associated with photon of known energy.

  34. Ex 3: a) How much energy is absorbed when a Hydrogen e- jumps from the n=1 to n=3 orbit?B) If the e- drops back down to the n=1 orbit, what f photon is emitted? C) To which type of radiation does that photon correspond?D) How many different photons are possible to be emitted by electron dropping from the n=3 to n=1 level?

  35. n =3 to n = 1 Ephoton = Einitial - Efinal. -13.6 eV - (-1.51 eV)= -12.1 eV (12.1 eV)(1.6 x 10-19 J/eV) = 1.936 x 10-18J.E = hf. f = E/hf = 1.936 x 10-18J/(6.63 x 10-34 Js)f = 2.92 x 1015 Hz. Look up.

  36. Ex 4: A Mercury Atom has an e- excited from the n=a to the n=e energy level. • What is the frequency it will absorb? • To which radiation does the frequency correspond? • If the e- drops down from the e to the b level, what type of radiation will it emit.

  37. Homework Set • Read Hamper 7.1 pay attention to purple box. Do 1 – 4 page 149 and • IB packet Bohr Model prb

  38. Go to Matter WavesNext PPT

  39. Einstein realized that matter contains energy. There is an equivalence of mass & energy.Energy is stored in the nucleus of atoms.The energy stored any mass obeys Einstein’s equation: E = energy in J.E = mc2. m = mass kg c = vel of light

  40. Ex 2: How much energy is produced when 2.5 kg of matter are completely converted to energy?How much energy is that in eV?

  41. E = mc2.=(2.5 kg )(3x108 m/s)2. = 2.25 x 1017 Jin eV(2.25 x 1017 J)(1 eV / 1.6 x 10 –19 J) = 1.4 x 1036 eV.

  42. Atomic Mass Units: amu or u • Mass of atoms very small so they are measured in amu or u. • Since mass is equivalent to energy, • 1 amu = 931 MeV or 931 x 106 eV.

  43. Ex 3: One universal atomic mass unit is equivalent to an energy of 931 MeV. Calculate the mass in kg of one universal mass unit.Hint: Use E = mc2 where energy is known in eV.

  44. Don’t forget to convert MeV to eV.(1 u) x (931 MeV/u) x (106eV/MeV) x (1.6 x 10 –19 J / eV) =1.49 x 1010 J E = mc2 so m = E/c2.(1.49 x 1010 J) / (3x108 m/s)2 = 1.66 x 10 –27 kg

  45. The mass units are based on the mass of a proton or 1H. (A hydrogen nucleus)

  46. Go to “The Nucleus” PPT

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