1. Bohr’s Model of the Atom 1913

2. Scientists noticed that the laws of Classical Physics that applied to large objects did not seem to be able to explain the behaviour of electrons giving rise to: Planck’s Quantum Theory The new experimental field of “Spectroscopy” demonstrated that atoms absorb and release bundles of energy. Bohr used both quantum theory and spectroscopy in formulating his model of the atom. Bohr’s model

3. Bohr’s model of the Atom Atoms and their electrons can only exist in certain specific Energy States. When moving within these allowed energy states (stationary states) the electron does NOT emit (release) energy.

4. Bohr’s model of the Atom For the Hydrogen atom: E n = -R /n 2 R = 2.18 x 10 -18 J/atom E 1 = -R /1 2 = - 21.8 x 10 -19 J/atom E 2 = -R /2 2 = - 5.44 x 10 -19 J/atom E 3 = -R /3 2 = - 2.42 x 10 -19 J/atom

5. Fixed Energy Levels n = Principal Quantum number; indicates the orbit.

6. Energy Levels in a Hydrogen atom

7. Why is there a negative sign in En? Bohr defined the point of zero potential energy as the point when the electron was infinitely removed from the atom.

8. Why is there a negative sign in En? When an electron moves closer to the nucleus, it feels an increasing attraction and the potential energy of the electron (and atom) decreases

9. Why is there a negative sign in En? When a free electron (at infinity) “falls” into E1 (n=1), 2.18 x 10 -18 J ( or 13.6 ev) is released.

10. Why is there a negative sign in En? For the Hydrogen atom: E1 = -R /1 E2 = -R /4 E3 = -R /9 Note that because of the negative sign, E1 is the lowest in energy (instead of the highest)

11. Electron Jumps Between Energy Levels 2.Each Energy level corresponds to an orbit. 3.The electron can travel in an orbit without radiating energy 4.An electron may only change its energy by jumping from one allowed level to another.

12. Electron Transitions-Excitation Energy Conversion: Radiant energy (EM radiation)  Potential energy (of electron) Electron jumps up a level

13. Electron Transitions ExcitationRelaxation Electron movesTo a higher E level Energy isAbsorbed Form of EnergyHeat, light, electrical Electromagnetic radiation

14. Electron Transitions-Relaxation Energy Conversion: Potential energy (of electron)  Radiant energy (EM radiation) Electron jumps down a level

15. Electron Transitions ExcitationRelaxation Electron movesTo a higher E levelTo a lower E level Energy isAbsorbedReleased Form of EnergyHeat, light, electrical Electromagnetic radiation

16. Energy of an atom or electron is “quantized” Atoms can only absorb or release the amount of energy necessary to move from one allowed energy level to another.

17. Excitation - Energy Absorbed n=1 n=2 n=4 n=3 n=5 n=6

18. Excitation - Energy Absorbed n=1 n=2 n=4 n=3 n=5 n=6 Absorbance of Energy (Excitation)

19. Relaxation - Energy Released n=1 n=2 n=4 n=3 n=5 n=6 Absorbance of Energy (Excitation) Emission of Energy (Relaxation)

20. Relaxation - Energy Released n=1 n=2 n=4 n=3 n=5 n=6 Absorbance of Energy (Excitation) Emission of Energy (Relaxation) E 2 to E 6 E 6 to E 2

21. Relaxation - Energy Released n=1 n=2 n=4 n=3 n=5 n=6 Absorbance of Energy (Excitation) Emission of Energy (Relaxation) E 2 to E 6 E 6 to E 2 BOTH TRANSITIONS INVOLVE THE SAME AMOUNT OF ENERGY. THUS INVOLVE THE SAME FREQUENCY OF EM RADIATION

22. What is observed during these transitions?

23. Background information: Electromagnetic energy (aka light energy) is often released and absorbed by chemical systems. The part of this radiation that we see is called the visible spectrum

24. Background information: Planck’s equation: E = hf States that the greater the frequency of the radiation, the greater the energy of the photons.

25. What happens to white light as it passes through a prism or diffraction grating (basis of spectroscopic instruments)? It is dispersed/separated according to the different colours (wavelengths, frequencies) Continuous spectrum

26. What happens to a sample of hydrogen gas (in a discharge tube) when an electric current is run through it? Electrical energy is absorbed and excites the electrons Relaxation follows H 2 emits a purplish light is emitted. Separating the light by a prism or diffraction grating produces…

27. Hydrogen Emission Spectrum Each coloured line is produced by a specific relaxation transition. Why do we only see 4 lines if many more transitions are possible?

28. Hydrogen Emission Spectrum Only the 4 transitions in the BALMER SERIES involve EM radiation in the VISIBLE range

29. Spectroscopy Analysis of the way matter absorbs or releases radiant energy Examples: IR-spectra, atomic emission spectra, NMR (nuclear magnetic resonance), absorption spectra

30. Spectroscopy is used to: a) identify elements or compounds Atomic Spectra: Fingerprints of Elements

31. Spectroscopy is used to: b) obtain information about bonding and structures of compounds IR spectrum

32. Spectroscopy is used to: c) Quantitatively determine the concentrations of substances present.

33. Two main types of atomic spectra: Emission and Absorption Spectra “Bright-line Spectra” is atomic emission spectra in the visible range.

34. Absorption vs. Emission Spectra of H2 Note that the spectral lines are on the same wavelength.

35. Advantages/Uses of Spectroscopy a) can definitively distinguish between substances with very similar physical and chemical properties. eg. Members of the alkali metals family KNa

36. Advantages/Uses of Spectroscopy b) Sample is distant Eg. Applications in Astronomy Astronomers have made the first direct detection and chemical analysis of an atmosphere of a planet that exists outside our solar system.

37. Spectrum of the hot gasesSpectrum of the hot gases in a nearby star- forming region, the Omega Nebula (M17)

38. Advantages/Uses of Spectroscopy b) Sample quantity is limited eg. in Forensics “Crystal meth”

39. The End