How Do We Know What We Can’t Even See?

1. How Do We Know What We Can’t Even See? Consider the atom as a “black box”.

2. Questions to ponder……. • How were scientists able to identify the parts of an atom? • When did these amazing discoveries take place? • What previous discoveries and technologies were necessary to “take apart” the atom?

3. Original Idea of “the atom” • Democritus (400 B.C.) • Atomos = indivisible • Dalton (1808) • Atom = indivisible sphere • Studied the ratios that elements combine

4. Dalton’s Atomic Theory • All matter is composed of extremely small particles called atoms. • Atoms of a given element are identical in size, mass and other properties; atoms of different elements differ in size, mass and other properties. • Atoms cannot be subdivided, created or destroyed. • Atoms of different elements combine in simple whole-number ratios to form chemical compounds. • In chemical reactions, atoms are combined, separated, or rearranged.

5. Dalton’s atom Indivisible sphere

6. J.J. Thomson’s CRT experiment • 1856 –1940; English physicist • Showed that a cathode ray would be deflected by an electric charge. http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/Disc-of-Electron-Images.html

7. Cathode Ray Experiment, cont… • Showedthat a cathode ray would be deflected by an electric charge. • The ray bent away from a negative plate • This was evidence that the ray was composed of negative particles. • The same results were observed no matter what type of gas was in the tube. • In another experiment, Thomson measured the charge-to-mass ratio of the cathode rays. • He found that the charge to mass ratio was very high. • So, the particles were either very light or very highly charged. • These particles were deemed “electrons”

8. http://www.chem.uiuc.edu/clcwebsite/cathode.html

9. Plum pudding model http://www.chemtopics.com/lectures/unit04/lecture1/l1u4.htm

10. Millikan’s Oil Drop Experiment • Robert Andrews Millikan (1868 –1953) • An American physicist • Won the 1923 Nobel Prize for measurement of the charge on the electron

11. Millikan’s Oil Drop Experiment • How did he do this? • He balanced the gravitational and electric forces on charged droplets of oil hovering between two metal electrodes.

12. Rutherford’s Gold Foil • Ernest Rutherford (1871 –1937) • A nuclear physicist from New Zealand. • Known as the "father" of nuclear physics.

13. Gold Foil Experiement

14. Rutherford animation • http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

15. Rutherford’s Gold Foil • Expected results: alpha particles passing through the plum pudding model of the atom undisturbed. • Bottom: Observed results: a small portion of the particles were deflected, indicating a small, concentrated positive charge.

16. Rutherford’s Gold Foil Rutherford concluded that the atom contained a very small positive charge which could repel the alpha particles if they came close enough. He proposed that the atom is mostly empty space, with most of the atom's mass concentrated in a tiny center, the nucleus.

17. Quick summary • Democritus atomos; no scientific evidence • Dalton atom smallest particle • Thomson  Cathode rays are composed of negative particles. Electrons. • Millikan Determined the charge of a single electron. • Rutherford  The atom has a dense center of positive charge. Nucleus.

18. Keep in mind • "If I have seen a little further it is by standing on the shoulders of Giants." - Isaac Newton • Each of these discoveries is the result of a chain of events and contributions from many, many scientists.