Atomic Structure and Function 25 September 2003 Professor Taylor

1. Atomic Structure and Function25 September 2003Professor Taylor Physics Chemistry Astronomy Geology Biology Ecology

2. Atomic Structure and Function:Outline • Early history in 18th and 19th centuries • Discovery of the electron (1897) • Discovery of the nucleus (1911) • Atomic structure: consternation • Atomic structure: the Bohr atom (1913) • Atomic Structure: Theory of Quantum Mechanics (1930’s) • Applications of our understanding • Discovery of the elements

3. Atomic Structure and Function:Take Home Message • Science as a evolutionary, incremental process over decades (e.g., exploring the structure of atom) • Role of hypothesis development and testing in the sciences – scientific method • Demystification of physics and chemistry: structure of the atom is a simple concept • Understanding of atomic structure underpins future course topics (e.g., chemical reactions, geology, biochemistry, ecology, environmental sciences) • Applications – some examples

4. Early History in 18th and 19th Century • Atomic Theory of Dalton • Atoms as indivisible “marbles” • Element (shape and size the same for a given element) • Molecules/compounds (e.g., water or H2O) combine in predictable ways and ratios • Atoms as basis for understanding chemistry but without electrons, protons and neutrons (analogy to a marble) • Elements – 60 known (e.g., hydrogen, gold, sodium)

5. Atomic Structure and Function:Outline • Early history in 18th and 19th centuries • Discovery of the electron (1897) • Discovery of the nucleus (1911) • Atomic structure with consternation • Atomic structure: the Bohr atom (1913) • Atomic Structure: Theory of Quantum Mechanics (1930’s) • Applications of our understanding

6. Thompson’s Discovery of the Electron • Idea that there may be some smaller components that comprise atoms • Hypothesis: Atoms consist of subcomponents, one of which is negatively charged and very small in mass • Experiment to test hypothesis

7. Thompson’s Experiment

8. Thompson’s Experiment

9. Thompson’s Discovery of the Electron • Idea that there may be some smaller components that comprise atoms • Hypothesis: Atoms consist of some subcomponents, one of which is negatively charged and very small in mass • Hypothesis: accept the hypothesis

10. Atomic Structure and Function:Outline • Early history in 18th and 19th centuries • Discovery of the electron (1897) • Discovery of the nucleus (1911) • Atomic structure with consternation • Atomic structure: the Bohr atom (1913) • Atomic Structure: Theory of Quantum Mechanics (1930’s) • Applications of our understanding

11. Rutherford’s Discovery of a Nucleus • Idea that there may be some smaller components that comprise atoms, including negatively and positively charges components • Hypothesis: Atoms consist of multiple subcomponents, some negative and some positive • Experiment to test hypothesis • Alpha particles (+), gold foil, “bullets”, and “tracks”

12. Rutherford’s Experiment:

13. Explanation

14. Observations, Hypothesis and Conclusion • Observations: • Almost all alpha’s passed through foil unaffected • Very small number of alpha’s deflected @ small angle • 1/1000 deflected at large angle (struck “head on”) • Hypothesis: Atoms consist of some positive subcomponents • Accept the hypothesis

15. New Model of Atom • Small, dense center, positively charged – called the nucleus (later to be called the proton) • Small, light (w/o mass), negative charged subcomponent circles the nucleus (electron) • Later additions (1932) • Neutrons • Protons • Charge (neutrality calls for + and – charges)

16. Atomic Structure and Function:Outline • Early history in 18th and 19th centuries • Discovery of the electron (1897) • Discovery of the nucleus (1911) • Atomic structure with consternation • Atomic structure: the Bohr atom (1913) • Atomic Structure: Theory of Quantum Mechanics (1930’s) • Applications of our understanding

17. Atomic Structure with Consternation Consternation with Rutherford Model • Electrons moving, so must give off energy • 2nd Law of Thermodynamics • Electrons should collapse • Yet atoms are billions of years old Conclusions: model violated fundamental laws of physics

18. Atomic Structure and Function:Outline • Early history in 18th and 19th centuries • Discovery of the electron (1897) • Discovery of the nucleus (1911) • Atomic structure with consternation • Atomic structure: the Bohr atom (1913) • Atomic Structure: Theory of Quantum Mechanics (1930’s) • Applications of our understanding

19. Atomic Structure: Bohr Atom • Observations (1913) • Heat hydrogen gas and light is emitted as a discrete wavelength (not continuous spectrum) • Other gases behave the same in producing discrete wavelength, but each gas unique in wavelengths produced

20. Atomic Structure: Bohr Atom • Hypothesis • Electrons circle the nucleus at specific distances from the nucleus in defined orbits • Diagram

21. Atomic Structure: Bohr Atom • Explanation of light in discrete wavelengths • As “excited”/heated electrons drop back from orbit, a packet of energy is lost as a function of distance from the nucleus • Packet of energy is called photon (light)

22. Atomic Structure: Bohr Atom • Key aspects of the Bohr atom • No space between the orbits for electrons to be so “jump” has to be “all or none” – quantum levels of electrons • Excited electrons in outer orbit has 2 options: • Drop down to next lower orbit and emit a packet of energy (light) unique for that distance • Drop back to even lower level (ground state) and emit packet of energy (light) unique for that that distance • Energy release is unique for each element and is a “window” on atomic structure (lab exercise) • Ideas verified two decades later using quantum mechanics

23. Bohr Atom

24. Atomic Structure and Function:Outline • Early history in 18th and 19th centuries • Discovery of the electron (1897) • Discovery of the nucleus (1911) • Atomic structure: consternation • Atomic structure: the Bohr atom (1913) • Atomic Structure: • Consternation with the Bohr Atom (theory) • Theory of Quantum Mechanics (1930’s) • Applications of our understanding • Discovery of the elements

25. Theory of Quantum Mechanics • Observations • Behavior of electrons in heated hydrogen gas were consistent with Bohr Model (orbits, etc.) • Behavior of other heavier gases could not be explained by Bohr Model • Investigators (Heisenberg, Shrodinger): wave-particle duality of light • Key: Integrated mechanisms of waves and particles, focusing on “fuzzy electron clouds”/waves • Hypothesis: new model of atomic structure and function: quantum mechanics theory of the atom • Incorporation of know facts • Prediction of new properties yet to be discovered

26. Quantum Mechanics

27. Quantum Mechanics

28. Quantum Mechanics Model • Key Principles • Electron behaves as a wave not a particle (Bohr) • Energy of electron behaves as a “spread-out wave” in three dimensions - not discrete bundle of energy (Bohr) • Description is based on quantum numbers • Distance from nucleus • Energy level and sublevel • Orientation of electrons in space • Direction of electron spin

29. Quantum Mechanics Model: Principle Energy Levels • Main energy level of electrons • Distance from nucleus • n = 1 (lowest energy level) • n = 2 • n = 3 • n = 4 (highest energy level) • etc. • Higher the number, the higher the energy

30. Quantum Mechanics Model: Sub-Energy Levels • Electrons in “fuzzy” probability distribution called an “orbital” • e.g., n = 1 has 1 sub-energy level/orbital (2 electron orientations) • Heisenberg Uncertainty Principle • Electrons in an orbital can only exist in pairs (2) and the pair spins in opposite directions • Pauli Exclusion Principle

31. Pauli’s Exclusion Principle

32. Quantum Mechanics Model: Electron Configuration • Electrons always adopt most energetically stable energy levels/orbitals consistent with Laws of Thermodynamics and previous conditions (above) • If n = 1, two electrons in 1st orbital (s orbital) • If n = 2, six electrons in 3 orbitals (p orbitals)

33. Electron Configuration in p Orbital

34. Atomic Structure and Function:Outline • Early history in 18th and 19th centuries • Discovery of the electron (1897) • Discovery of the nucleus (1911) • Atomic structure with consternation • Atomic structure: the Bohr atom (1913) • Atomic Structure: Theory of Quantum Mechanics (1930’s) • Applications of our understanding

35. Applications of Our Understanding of Atomic Structure • Spectroscopy: quantum differences in light emitted or absorbed result in unique signature for each element (“fingerprint”) • LASER (Light Amplification by Stimulated Emission of Radiation) • Binding of elements to create compounds • Neurotransmitters in the brain • Allergy reactions • Etc.

36. Atomic Structure and Function:Take Home Message • Science as a evolutionary, incremental process over decades (e.g., exploring the structure of atom) • Role of hypothesis development and testing in the sciences – scientific method and progress • Consternation, rejection, new models & new hypotheses • Demystification of physics and chemistry: structure of the atom is a simple concept • Understanding of atomic structure underpins future course topics (e.g., chemical reactions, geology, biochemistry, ecology, environmental sciences) • Applications – some examples

37. Atomic Structure

38. Atomic Structure and Function:Outline • Early history in 18th and 19th centuries • Discovery of the electron (1897) • Discovery of the nucleus (1911) • Atomic structure: consternation • Atomic structure: the Bohr atom (1913) • Atomic Structure: Theory of Quantum Mechanics (1930’s) • Applications of our understanding • Discovery of the elements