1P22/1P92 (2011) Chapter 29 Atoms and Molecules Friday, January 14, 2011 10:03 AM What is the inside of an atom like? Geiger-Marsden experiment (1909) Rutherford's atomic model (1911) --- a good step, but it was NOT quantitative • one of the lessons/methods of science: look for the unexpected Also, Rutherford's model suffered from a fatal flaw: according to classical electromagnetic theory (Maxwell's equations), atoms should emit continuous (that is, a wide range of wavelengths) electromagnetic radiation (they don't; rather they emit "line spectra"), and they should be unstable (electrons should lose energy as they emit electromagnetic radiation, and eventually spiral into the nucleus, where they would die a dramatic death upon joining forces with the protons in the nucleus), which they aren't. So, although it was clear that Rutherford was onto something, it was also very clear that his model was wrong. Enter Niels Bohr. - Bohr's atomic model; see pages 960 ff in the textbook What are line spectra? • emission and absorption spectra: Ch29P Page 1 Ch29P Page 2 Ch29P Page 3 Ch29P Page 4 Ch29P Page 5 Ch29P Page 6 Pasted from <http://www.google.ca/imgres?q=electron+orbitals&um=1&sa=N&biw=1280&bih=576&hl=en&tbm=isch&tbnid=1zKh-g8udAwM6M: &imgrefurl=http://chemicalfacts4u.blogspot.com/2011/06/atomic-orbital.html&docid=F81z3i_N2aTcM&imgurl=http://3.bp.blogspot.com/-7WaPCpa9mF4/TfgpuFxlFII/AAAAAAAAACA/x5_0HjUpn2Y/s1600/ch9orbitals1.jpg&w=576&h=388 &ei=yyLcUej6Jq7BywHh2oH4BQ&zoom=1&iact=hc&vpx=721&vpy=165&dur=328&hovh=184&hovw=274&tx=168&ty=107&page=1&tbnh=136&tbnw= 191&start=0&ndsp=18&ved=1t:429,r:4,s:0,i:98> Ch29P Page 7 Ch29P Page 8 Exercises CP 3 The Paschen series is analogous to the Balmer series, but with m = 3. Calculate the wavelengths of the first three members in the Paschen series. Which part(s) of the electromagnetic spectrum are these in? Ch29P Page 9 CP 10 The allowed energies of a simple atom are 0.0 eV, 4.0 eV, and 6.0 eV. (a) Draw the atom's energy-level diagram. Label each level with the energy and the principal quantum number. (b) Which wavelengths appear in the atom's emission spectrum? (c.) Which wavelengths appear in the atom's absorption spectrum? Ch29P Page 10 CP 12 A researcher observes hydrogen emitting photons of energy 1.89 eV. What are the quantum numbers of the two states involved in the transition that emits these photons? Consider instead the related problem where we use CP 13 A hydrogen atom is in the n = 3 state. In the Bohr model, how many electron wavelengths fit around this orbit? Ch29P Page 11 Ch29P Page 12 Ch29P Page 13 Ch29P Page 14 - the idea of electron energy levels and "shells" was later adapted to help describe atomic nuclei as well Ch29P Page 15 Exercises CP 26 Predict the ground-state electron configurations of Mg, Sr, and Ba. CP 31 Explain what is wrong with each electron configuration. (a) 1s22s22p83s23p4 (b) 1s22s32p4 CP 30 Identify the element for each electron configuration. Then determine whether this configuration is the ground state or an excited state. (a) 1s22s22p63s23p64s23d9 (b) 1s22s22p63s23p64s23d104p65s24d105p66s24f145d7 Ch29P Page 16 CP 34 Hydrogen gas absorbs light of wavelength 103 nm. Afterward, what wavelengths are seen in the emission spectrum? Ch29P Page 17