2. 1 ATOMS: Dalton and Beyond A search for a simple theory of matter Topic 7 – Spring 2006 Ted Georgian, Dept. of Biology

3. 2 The nature of science Scientists are searching for explanations that are: 1.2.3.

4. 3 What is the world made of, at the most fundamental level? www.ikros.net/mocs/ images/Castle2Clouds.jpg

5. 4 Early Greek atomists Democritus (470 - 380 B.C.) www.livius.org/a/ 1/greeks/democritus.jpg Leucippus (~480 - 420 B.C.) http://cont1.edunet4u.net/cobac2/down/dow n05.html All matter is made of tiny, indivisible particles called “atoms” Change is caused by atoms moving through empty space (a “void”) Atoms are therefore “fundamental”

6. 5 But an alternate model won out http://astsun.astro.virginia.edu/~jh8h/Foundations/ch apter2.html Aristotle (384 – 322 BC) Based on observable characteristics of matter, such as ?

7. 6 The mechanical philosophy of the 1600s Descartes, Boyle, Newton & others imagined a “clockwork” universe - perfectly predictable

8. 7 Would it work for chemistry as well? Maybe chemistry would turn out to be as “simple” as Newtonian physics? A few, simple objects following simple, general, and precise laws.

9. 8 Start of the Modern Era of Chemistry John Dalton’s Atomic Hypothesis (1808): 1.All matter is made up of indivisible atoms. 2.Compounds are composed of atoms in definite proportions. 3.Chemical change occurs when atoms are rearranged

10. 9 Dalton’s Atomic Model of Compounds explained observation of “constant proportions” used hypothesis (“Rule”) of greatest simplicity estimated relative atomic masses

11. 10 Meanwhile, many new elements being found

12. 11 How to make sense of all these elements? Scientists like “a place for everything, and everything in its place.” And no more places and things than necessary.

13. 12 Dmitri Mendeleev (1834-1907) “Creator of the Periodic Table ” (but there were earlier attempts by others)

14. 13 Mendeleev’s early notes for the Periodic Table (1869)

15. 14 Characteristics of Mendeleev’s Table Organized 60+ known elements… - by similar chemical properties in each vertical family (group) - by roughly increasing atomic weight within each horizontal row Used to predict existence of new elements (of 10, found 7; other 3 do not exist)

16. 15 Mendeleev’s table, as originally published Formatted sideways compared to modern table ? instead of a name: element was predicted to exist but not known yet

17. 16 Prediction of the properties of an unknown element below Silicon PropertyObserved for Si Predicted for eka-Si Observed for Sn Atomic mass 2872118 Density (g/cm 2 ) 2.335.57.28 Formula of oxide SiO 2 Eka-SiO 2 SnO 2 Formula of chloride SiCl 4 Eka-SiCl 4 SnCl 4 eka: “one beyond ” * Observed for Ge 72.6 5.35 GeO 2 GeCl 4

18. 17 An attempt to simplify the elements William Prout (1815) hypothesized that the hydrogen atom is fundamental all other elements made up of hydrogen atoms his hypothesis was rejected by the 1830s (for ex. chlorine atom had mass 35.4 times that of hydrogen)

19. 18 News flash: a new type of matter is discovered J. J. Thomson (1897) experimented with “cathode rays” decided that they are charges of electricity carried by particles of matter Schematic of actual 1897 apparatus (vacuum inside):

20. 19 Cathode-Ray Tubes – ever seen one? http://www.howstuffworks.com/tv4.htm

21. 20 Thomson’s conclusions but... “What are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision?” - J. J. Thomson http://www.aip.org/history.electron/jjrays.htm “We have, in the cathode rays, matter in a new state...a state in which all matter...is of one and the same kind; this matter being the substance from which all the chemical elements are built up."

22. 21 How big are “electrons”? Thomson calculated the mass-to-charge ratio for cathode ray particles: it was over 1000 times smaller than of a charged hydrogen atom This fact suggested: - either cathode rays carried a huge charge, - or they had very small mass

23. 22 Answer: very, very small Robert Millikan measured the charge of a cathode ray particle in 1910. From that & Thompson’s mass- to-charge ratio, he could calculate the mass: ~1800 times lighter than a hydrogen atom

24. 23 Thomson’s “plum pudding” atom model * * Never had plum pudding? Think of a blueberry muffin. tiny “corpuscles” of negative charge surrounded by a sort of “cloud” of positive charge

25. 24 More pieces of the atom Ernest Rutherford Thomson’s student Gold Leaf Experiment (1910-11) – actually conducted by Hans Geiger and undergraduate Ernest Marsden

26. 25 The gold leaf experiment fired positively-charged alpha particles at very thin gold foil – they caused flashes of light when they hit the screen counted flashes and measured the angle of deflection

27. 26 Gold leaf experiment: prediction By Thomson’s model, mass and + charge of gold atom are too dispersed to deflect the positively-charged alpha particles, so... particles should shoot straight through the gold atoms.

28. 27 like this:

29. 28 What actually happened:

30. 29 What’s going on? Most alpha particles went straight through, and some were deflected, BUT a few (1 in 20,000) reflected straight back to the source! “It was quite the most incredible event that has ever happened to me. It was almost as incredible as if you had fired a fifteen inch shell at a piece of tissue paper and it came back and hit you.”

31. 30 Rutherford’s Model of the Atom small, dense, positively- charged nucleus surrounded by “mostly empty” space in which the electrons must exist. positively charged particles called “protons” like tiny solar system +

32. 31 The nucleus repels alpha particles +

33. 32 How much of an atom is empty space? In fact, if the nucleus of an atom were the size of Murphy Auditorium, the innermost electrons would be how far away? A.DeLaRoche Hall? B.Francis Hall? C.Downtown Olean? D.NYC? + Most of it! (click for the right answer)

34. 33 But wait – there’s more! James Chadwick (1932) discovered a neutral (uncharged) particle in the nucleus. called it the “neutron”

35. 34 Atom “split” later that year Atom “split” by John Cockcroft and Ernest Walton, using a particle accelerator, in late 1932

36. 35 Splitting the atom led to some very practical consequences

37. 36 Now we understand why the periodic table works The order of the elements is determined by their atomic number (= the number of protons) The atomic mass of the elements is determined by the number of protons and neutrons. The chemical properties of the elements are determined by the number of electrons in their outer (valence) shells

38. 37 Why do 2 Group I atoms combine with 1 oxygen (R 2 O)?

39. 38

40. 39 So: is this what atoms are like? No! Calculations soon showed that a “Rutherford atom” would last less than one minute Electrons would radiate away energy and spiral down into the nucleus.

41. 40 A new understanding of the atom from spectroscopy When elements are heated, they give off light of a particular wavelength (or color) Sodium Potassium Lithium

42. 41 Spectroscopes: seeing atomic light Original 1859 Bunsen- Kirchhoff spectroscope Modern apparatus for viewing a “spectrum”

43. 42 Hydrogen’s emission “fingerprint” The line-emission spectrum of hydrogen gas Observation: when heated with electricity hydrogen gives off light of specific wavelengths

44. 43 Niels Bohr (1885-1962) Danish physicist Bohr wondered why hydrogen emitted spectral lines, and not just a continuous band of light

45. 44 Bohr’s Model of Atom (1913) Bohr assumed that electrons can orbit ONLY at certain distances from nucleus this model permits electrons to exist for a long time without giving off radiation Bohr’s model enabled him to predict the number and wavelength of hydrogen’s emission lines

46. 45 Electron orbits are distinct (“quantized”) in Bohr’s model Trefil & Hazen. The Sciences: An integrated approach. 2 nd ed. Fig. 7-6. “ Quantum leaps” from one level to another

47. 46 But why should electrons behave this way? “Thus I arrived at the following general idea …: for matter, just as much as for radiation, in particular light, we must introduce at one and the same time the corpuscle concept and the wave concept.” http://www.spaceandmotion.com/Physics-Louis-de-Broglie.htm Louis de Broglie (1927) Particle/Wave Duality of electrons

48. 47 Electrons as waves Only at certain distances from the nucleus would an electron complete an integer number of wavelengths in its orbit When de Broglie did the mathematics, he could predict exactly the distances that Bohr had assumed for the hydrogen atom.

49. 48 Then, suddenly, trouble for the mechanistic approach Werner Heisenberg (1927) The “Uncertainty Principle” There’s an upper limit to how precisely an electron’s position and momentum can be known The more precisely one is known, the less precisely the other can be known

50. 49 Electrons move in “probability clouds”, not circular orbits The exact path of an electron can’t be predicted! If we know the electron is in a given atom, its velocity is uncertain by ~16 million mph!!

51. 50 Newtonian certainty cannot be obtained in the subatomic world “I cannot believe that God plays dice with the universe.” “Albert, stop telling God what to do.”

52. 51 Here we go again! By the 1950s hundreds of sub-atomic particles had been identified. Simplicity was getting lost again.

53. 52 Another attempt to simplify our model of matter Murray Gell-Mann and George Zweig (1964) - proposed that protons and neutrons are made of smaller particles they named quarks (aces)

54. 53 Gell-Mann & Zweig hypothesized 6 different quarks Quarks have fractional charges Then how do particles in the nucleus end up with +1 or zero charges?

55. 54 Protons & neutrons are not fundamental Protons and neutrons are composed of UP and DOWN quarks, held together by gluon particles

56. 55 Fermi National Accelerator Lab: 6-km Tevatron ring and 3-km Main Injector * Chicago site for study of sub- subatomic particles Evidence for last quark (TOP) found in 1995 *contrast to world’s-largest machine: CERN 27-km LEP collider (1989-2000)

57. 56 So: are quarks fundamental? Probably not: recent models of matter hypothesize 11-dimensional “strings” curled up inside of quarks …stay tuned for future developments.