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How do we know the structure of the atom?

How do we know the structure of the atom?. The famous Geiger-Marsden Alpha scattering experiment. In 1909, Geiger and Marsden were studying how alpha particles are scattered by a thin gold foil. Thin gold foil. Alpha source. Geiger-Marsden.

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How do we know the structure of the atom?

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  1. How do we know the structure of the atom?

  2. The famous Geiger-Marsden Alpha scattering experiment In 1909, Geiger and Marsden were studying how alpha particles are scattered by a thin gold foil. Thin gold foil Alpha source

  3. Geiger-Marsden As expected, most alpha particles were detected at very small scattering angles Thin gold foil Small-angle scattering Alpha particles

  4. Geiger-Marsden To their great surprise, they found that some alpha particles (1 in 20 000) had very large scattering angles Thin gold foil Small-angle scattering Alpha particles Large-angle scattering

  5. Explaining Geiger and Marsdens’ results The results suggested that the positive (repulsive) charge must be concentrated at the centre of the atom. Most alpha particles do not pass close to this so pass undisturbed, only alpha particles passing very close to this small nucleus get repelled backwards (the nucleus must also be very massive for this to happen). nucleus

  6. Rutherford did the calculations! Rutherford (their supervisor) calculated theoretically the number of alpha particles that should be scattered at different angles. He found agreement with the experimental results if he assumed the atomic nucleus was confined to a diameter of about 10-15 metres.

  7. Try this problem In a Geiger Marsden scattering experiment, a nucleus has a diameter of (m) 1.4 x 10-14 The alpha particle has a mass of (kg) 6.64 x 10-27 What is the kinetic energy of the alpha particle that has a de Broglie wavelength equal to the diameter of the nucleus? How fast is it traveling?

  8. And this one The same alpha particle is fired at a gold nucleus (Z=79) How close does the alpha particle get to the nucleus?

  9. Rutherford did the calculations! That’s 100 000 times smaller than the size of an atom(about 10-10 metres).

  10. Stadium as atom If the nucleus of an atom was a ping-pong ball, the atom would be the size of a football stadium (and mostly full of nothing)! The model has electrons orbiting like planets. Nucleus (ping-pong ball

  11. Limitations of this model? • According to the theory of electromagnetism, an accelerating charge (and the orbiting electrons ARE accelerating centripetally) should radiate energy and thus spiral into the nucleus.

  12. Evidence for atomic energy levels

  13. Light emitted cathode anode Low pressure gas electric current Evidence for atomic energy levels When a gas is heated to a high temperature, or if an electric current is passed through the gas, it begins to glow.

  14. Emission spectrum If we look at the light emitted (using a spectroscope) we see a series of sharp lines of different colours. This is called an emission spectrum.

  15. Absorption Spectrum Similarly, if light is shone through a cold gas, there are sharp dark lines in exactly the same place the bright lines appeared in the emission spectrum. Light source gas Some wavelengths missing!

  16. The Rydberg Equation Balmer and Rydberg (this version) came up with a formula to show all these energies, but no explanation as to why: Where R is the Rydberg constant 1.096 x 107 m-1 And the energy is E = hc/l

  17. Answer this one: What is the energy of a photon from the Rydberg Equation using n=3?

  18. Why? Scientists had known about these lines since the 19th century, and they had been used to identify elements (including helium in the sun), but scientists could not explain them.

  19. Niels Bohr In 1913, a Danish physicist called Niels Bohr realised that the secret of atomic structure lay in its discreteness, that energy could only be absorbed or emitted at certain values. At school they called me “Bohr the Bore”!

  20. The Bohr Model Bohr realised that the electrons could only be at specific energy levels (or states) around the atom.

  21. The Bohr Model We say that the energy of the electron (and thus the atom) can exist in a number of states n=1, n=2, n=3 etc. (Similar to the “shells” or electron orbitals that chemists talk about!) n = 1 n = 2 n = 3

  22. High energy n levels are very close to each other n = 5 n = 4 n = 3 n = 2 n = 1 (the ground state) The Bohr Model The energy level diagram of the hydrogen atom according to the Bohr model 0 Energy eV Electron can’t have less energy than this -13.6

  23. 0 Energy eV n = 5 n = 4 n = 3 n = 2 -13.6 n = 1 (the ground state) The Bohr Model An electron in a higher state than the ground state is called an excited electron. High energy n levels are very close to each other electron

  24. 0 Energy eV n = 5 n = 4 n = 3 n = 2 -13.6 n = 1 (the ground state) Atomic transitions If a hydrogen atom is in an excited state, it can make a transition to a lower state. Thus an atom in state n = 2 can go to n = 1 (an electron jumps from orbit n = 2 to n = 1) Wheeee! electron

  25. 0 Energy eV n = 5 n = 4 n = 3 n = 2 -13.6 n = 1 (the ground state) Atomic transitions Every time an atom (electron in the atom) makes a transition, a singlephoton of light is emitted. electron

  26. 0 Energy eV n = 5 n = 4 n = 3 n = 2 -13.6 n = 1 (the ground state) Atomic transitions The energy of the photon is equal to the difference in energy (ΔE) between the two states. It is equal to hf. ΔE = hf electron ΔE = hf

  27. 0 Energy eV n = 5 n = 4 n = 3 n = 2 -13.6 n = 1 (the ground state) The Lyman Series Transitions down to the n = 1 state give a series of spectral lines in the UV region called the Lyman series. Lyman series of spectral lines (UV)

  28. 0 Energy eV n = 5 n = 4 n = 3 n = 2 -13.6 n = 1 (the ground state) The Balmer Series Transitions down to the n = 2 state give a series of spectral lines in the visible region called the Balmer series. Balmer series of spectral lines (visible) UV

  29. 0 Energy eV n = 5 n = 4 n = 3 n = 2 -13.6 n = 1 (the ground state) The Pashen Series Transitions down to the n = 3 state give a series of spectral lines in the infra-red region called the Pashen series. Pashen series (IR) visible UV

  30. Emission Spectrum of Hydrogen The emission and absorption spectrum of hydrogen is thus predicted to contain a line spectrum at very specific wavelengths, a fact verified by experiment. Which is the emission spectrum and which is the absorption spectrum?

  31. 0 Energy eV n = 5 n = 4 n = 3 n = 2 -13.6 n = 1 (the ground state) Pattern of lines Since the higher states are closer to one another, the wavelengths of the photons emitted tend to be close too. There is a “crowding” of wavelengths at the low wavelength part of the spectrum Spectrum produced

  32. How do you excite an atom? • Heating to a high temperature • Bombarding with electrons • Having photons fall on the atom I’m excited!

  33. Limitations of the Bohr Model • Can only treat atoms or ions with one electron • Does not predict the intensities of the spectral lines • Inconsistent with the uncertainty principle (see later!) • Does not predict the observed splitting of the spectral lines

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