1. e-e- e-e- Ground state Excited state Electrons can only be at specific energy levels, NOT between levels.

2. Color = Energy of Photons Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 329

3. Energy Level Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 329 A B CD Ground state Energy Four excited states

4. An Excited Lithium Atom Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 326 Photon of red light emitted Li atom in lower energy state Excited Li atom Energy

5. Excitation of Hydrogen Atoms Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 328

6. Return to Ground State

7. Frequency A Frequency B Frequency C n = 2 n = 1 n = 3

8. Frequency A Frequency B Frequency C n = 2 n = 1 n = 3 A B C A + B = C

9. Bohr Model of the Atom Electrons in Atoms nucleus (+) electron (-) Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

10. Bohr Model electrons exist only in orbits with specific amounts of energy called energy levels Therefore… electrons can only gain or lose certain amounts of energy only certain photons are produced Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

11. Bohr Model 1 2 3 4 5 6 Energy of photon depends on the difference in energy levels Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem nucleus

12. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. (a) Electronic absorption transition (b) H 2 emission spectrum (top), H 2 absorption spectrum (bottom)

13. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. (a) Electronic absorption transition (b) H 2 emission spectrum (top), H 2 absorption spectrum (bottom) Lower-energy orbit Higher-energy orbit Photon e-e- e-e-

14. Bohr Atom The Planetary Model of the Atom

15. Bohr’s Model Nucleus Electron Orbit Energy Levels

16. Bohr Model of Atom The Bohr model of the atom, like many ideas in the history of science, was at first prompted by and later partially disproved by experimentation. http://en.wikipedia.org/wiki/Category:Chemistry Increasing energy of orbits n = 1 n = 2 n = 3 A photon is emitted with energy E = hf e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e-

17. Cartoon courtesy of NearingZero.net

18. Development of the Atom

19. Planetary Model +

20. An unsatisfactory model for the hydrogen atom According to classical physics, light should be emitted as the electron circles the nucleus. A loss of energy would cause the electron to be drawn closer to the nucleus and eventually spiral into it. Hill, Petrucci, General Chemistry An Integrated Approach 2nd Edition, page 294

21. Quantum Mechanical Model Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions of space (orbitals). Niels Bohr & Albert Einstein

22. Development of Atomic Models Rutherford model In the early twentieth century, Rutherford showed that most of an atom's mass is concentrated in a small, positively charged region called the nucleus. Bohr model After Rutherford's discovery, Bohr proposed that electrons travel in definite orbits around the nucleus. Thomson model In the nineteenth century, Thomson described the atom as a ball of positive charge containing a number of electrons. Quantum mechanical model Modern atomic theory described the electronic structure of the atom as the probability of finding electrons within certain regions of space.

23. Modern View The atom is mostly empty space Two regions – Nucleus protons and neutrons – Electron cloud region where you might find an electron