ATOMS: Dalton and Beyond A search for a simple theory of matter Topic 7 – Spring 2005

1. ATOMS: Dalton and BeyondA search for a simple theory of matterTopic 7 – Spring 2005 Ted Georgian, Dept. of Biology

2. The nature of science Scientists are searching for explanations that are: 1. 2. 3.

3. Models in science Scientists use their experimental results (and their imaginations) to create models A model is a representation of a complex natural system that permits us to understand its behavior. These models may be mental, mathematical, or even physical.

4. Prior Examples? Remember the models of planetary motion? What were they trying to explain? (These animated gifs were obtained from Dr. Stephen J. Daunt'sAstronomy 161 web site at The University of Tennesee, Knoxville.)

5. There is only one type of matter, found in tiny, indivisible particles called “atoms” • All change is caused by atoms moving through empty space • Atoms are therefore “fundamental” Leucippus (~480 - 420 B.C.) http://cont1.edunet4u.net/cobac2/down/down05.html Early Greek atomists Democritus (470 - 380 B.C.) www.livius.org/a/ 1/greeks/democritus.jpg

6. But an alternate model won out http://astsun.astro.virginia.edu/~jh8h/Foundations/chapter2.html Aristotle (384 – 322 BC)

7. Descartes, Boyle and Newton • A “clockwork” universe • Simplicity, generality, and precise predictions The mechanical philosophy of the 1600s

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

9. Start of the Modern Era of Atoms John Dalton’s Atomic Hypothesis (1803): • All matter is made up of indivisible atoms. • Compounds are composed of atoms in definite proportions. • Chemical change occurs when atoms are rearranged

10. Dalton’s Atomic Model of Compounds • explained observation of “constant proportions” as based on atomic composition of compounds • used “Rule of greatest simplicity” to guess at the atomic structure of compounds • estimated relative atomic masses, based on his hypothesized structures

11. Meanwhile, many new elements being found

12. 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. Dmitri Mendeleev (1834-1907) “Creator of the Periodic Table” (but there were earlier attempts by Dobereiner and Newlands, and Meyer probably formulated the periodic idea at same time as Mendeleev)

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

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

16. Characteristics of Mendeleev’s Table • Organized 60+ known elements… - by similar propertiesin each verticalfamily (group) - by roughly increasing atomic weight within each horizontal row (moved 17 elements based on properties rather than weight) • Used to predict existence of new elements (of 10, found 7; other 3 do not exist)

17. Prediction of the properties of an unknown Group 4 element below Silicon * eka: “one beyond”

18. 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. News flash: atoms aren’t fundamental J. J. Thomson (1897) • experimented with “cathode rays” • “and then... made a bold speculative leap. Cathode rays are not only material particles, he suggested, but in fact the building blocks of the atom: they are the long-sought basic unit of all matter in the universe.” (http://www.aip.org/history/electron/jjrays.htm) Schematic of actual 1897 apparatus (vacuum inside):

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

21. Thomson’s conclusions • “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." • “I can see no escape from the conclusion that [cathode rays] are charges of electricity carried by particles of matter.” but... • “What are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision? - J. J. Thomson

22. Cathode rays (electrons) are... • tiny “corpuscles” of negative charge • surrounded by a sort of “cloud” of positive charge Thomson’s “plum pudding” atom model* * Never had plum pudding? Think of a blueberry muffin.

23. If electrons exist, how big are they? • Thomson calculated the mass-to-charge ratio for cathode ray particles: it was over 1000 times smaller than for a charged hydrogen atom • This fact suggested: - either cathode rays carried a huge charge, - or they had very small mass • Robert Millikan measured the charge of a cathode ray particle in 1910. From that he could calculate the mass: ~1800 times lighter than a hydrogen atom

24. More pieces of the atom Ernest Rutherford (1871-1937) nuclear physicist, Thomson’s student, New Zealander teaching in Great Britain Gold Leaf Experiment

25. Rutherford’s Experiments (1910-11)(done by undergrad Ernest Marsden/physicist Hans Geiger) • Fired beam of positively-charged alpha particles at very thin gold foil. • Alpha particles caused flashes of light when they hit the zinc sulfide screen

26. Rutherford’s 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.

27. Rutherford’s Experiment: prediction Alpha particles will pass through like this …

28. Rutherford’s experiment: what actually happened

29. Most alpha particles went straight through, and someweredeflected, BUT a few(1 in 20,000)reflectedstraight back to the source! What’s going on? “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.”

30. Expt. Interpretation: • gold atom has small, dense, positively-chargednucleus surrounded by “mostly empty” space • in which the electrons must exist. • Positively charged particles called “protons” • like tiny solar system + Rutherford’s Model of the Atom

31. The Nucleus Repels Alpha Particles +

32. Most of it! • In fact, if the nucleus of an atom were the size of a marble, the innermost electrons would be how far away? • One-half inch • Six inches • Eighteen inches • One-half mile + How much of an atom is empty space? (click for the right answer)

33. But wait – there’s more! James Chadwick (1932) Discovered a neutral (uncharged) particle in the nucleus. Called it the “neutron” Atom “split” by John Cockcroft and Ernest Walton, using a particle accelerator, in late 1932

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

35. Splitting the atom led to some very practical consequences

36. Properties of Subatomic Particles

37. Now we understand why the elements come in periods of 8 • 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. A given element can have different number of neutrons, and therefore different atomic masses. • The chemical properties of the elements are determined by the number of electrons in their outer (valence) shells

38. Why do 2 Group I atoms combine with 1 oxygen (R2O)?

39. Modern Periodic Table Organization Elements are NOW placed in order of increasing atomic number (#of + protons). - Why? Gives absolute order... atomic weights not characteristic (different-mass atoms called isotopes exist!) A relationship between nuclear charge and arrangement of elements in the Table was finally discovered in1914 (Henry Moseley). In 1860s, Mendeleev could NOT have predicted a relationship to subatomic particles!

40. So: is this what atoms are like? No! Electrons moving through the electrical field generated by the protons in the nucleus would radiate away energy and spiral down into the nucleus Calculations soon showed that a “Rutherford atom” would last less that one minute.

41. A new understanding of the atom It had long been know that when chemical elements are heated, they gave off light of a particular wavelength (or color) Sodium Potassium Lithium

42. Spectroscopes: Seeing Atomic Light Original 1859 Bunsen- Kirchhoff spectroscope Typical setup for viewing a line-emission spectrum

43. Spectroscopy can identify elements on distant stars HydrogenHelium http://heasarc.gsfc.nasa.gov/docs/xmm_lc/edu/lessons/student-worksheet-spectragraph2.html Helium was discovered in a spectrograph of the Sun in 1868 and not on Earth until 27 years later.

44. Hydrogen’s Emission “Fingerprint” Observation: when hit with electricity hydrogen gives off light of specific wavelengths, NOTcontinuous range! The line-emission spectrum of hydrogen gas (the bands visible to humans)

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

46. Bohr’s Model of Atom (1913) • Circling electron maintains orbit ONLYatspecific distances from nucleus • Only way electron could exist for long time without giving off radiation • Bohr’s model enabled him to predict the number and wavelength of hydrogen’s emission lines

47. Electron orbits are distinct (“quantized”) in Bohr’s model Trefil & Hazen. The Sciences: An integrated approach. 2nd ed. Fig. 7-6.

48. But why should electrons behave this way? Louis de Broglie(1927) Particle/Wave Duality of electrons Thus I arrived at the following general idea which has guided my researches: 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. In other words, in both cases we must assume the existence of corpuscles accompanied by waves. De Broglies Nobel Prize speech, 1927. http://www.spaceandmotion.com/Physics-Louis-de-Broglie.htm

49. Electrons can be thought of as standing waves …

50. Electrons as waves Only at certain distances from the nucleus would the electron complete an integer number of wavelengths in its movement around the nucleus When the mathematics was worked out, these distances agreed exactly with those assumed by Bohr for the hydrogen atom.