Atomic Structure

1. Atomic Structure Cause of Electrostatics Force & Electricity

2. Atom History • Atom Comes from “atomos” • Democritus first developed the theory of an indivisible part of matter • Survived for almost 2000 years

3. Greek Model • Atoms were spherical particles that are indivisible forms of matter • Water atoms shown below according to Greek model

4. Dalton Model for Atom • Stated “Matter, though divisible in an extreme degree, is nevertheless not infinitely divisible” • Was the first experimental evidence for the atomic theory produced nearly 2000 years earlier • Atoms still thought to be indivisible

5. Daltons Experiments

6. Dalton’s Experiments • First experimental evidence of atomic theory • Held for almost 100 years more as indivisible atoms

7. Thomson Experiment • Studied the cathode rays that are produced when electricity is put in a glass tube • Cathode rays move toward positive charge

8. Cathode Ray Tube • Animation • http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::100%::100%::/sites/dl/free/0072512644/117354/01_Cathode_Ray_Tube.swf::Cathode%20Ray%20Tube

9. Thomson • cathode rays were over 1000 times lighter than the hydrogen atom. comparing the deflection of a beam of cathode rays by electric and magnetic fields he was then able to get more robust measurements of the mass to charge ratio that confirmed his previous estimates.

10. Thomson Plum Pudding Model • Proved atoms must have a negative charge in them • If total charge of an atom is zero then there must be positive charge somewhere • Held for only 15 years

11. Rutherford Experiment • Fired alpha particles at a very thin gold foil • Measured the angle the rays deflected as they passed thru

12. Rutherford’s Gold Foil Experiment Top: 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 charge. Note that the image is not to scale; in reality the nucleus is vastly smaller than the electron shell.

13. Rutherford Experiment Close Up

14. Alpha, Beta & Gamma Particles • Animation • http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::100%::100%::/sites/dl/free/0072512644/117354/rev03_Alpha_Beta_Gamma_Rays.swf::Alpha%20Beta%20Gamma%20Rays

15. Rutherford Model • Atoms are made of mostly empty space • Similar to how planets orbit a star

16. Niels Bohr • Worked closely with Rutherford after Rutherford’s famous experiment • Tried to explain line spectrums or electromagnetic waves produced when sealed gas tubes are hooked up to electricity

17. Line Spectrum • Different atoms produce different isolated spectrum

18. Millikan • Experimented with oil droplets to determine charge on electron. • Animation • http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::100%::100%::/sites/dl/free/0072512644/117354/02_Millikan_Oil_Drop.swf::Milikan%20Oil%20Drop

19. Proton-Neutron-Electron Size The Proton and the neutron are roughly equal size with a neutron being slightly biggerand slightly more massive. Compared to one neutron’s mass, a proton would be 99.86% the mass. An electron would be 0.054% the mass of a neutron. The electron is roughly 1000 X smaller than both

20. MegaElectron Volts • Another useful measure of the relative mass of the subatomic particles is to convert them to energy using Einstein’s most famous equation e=mc2. • This gives us energy in Joules, but for energies at these tiny scales it is more useful to use electron volts (eV) or MegaElectron Volts (MeV) • 1 eV = 1.6 x 10-19 Joules (remember that value?) • Neutron = 939.6 MeV • Proton = 938.3 MeV • Electron = 0.51 MeV

21. Proton-Electron Distance The proton-electron distance is about 100,000 times the diameter of a proton or about 100,000,000 times the diameter of an electron Earth to beyond Pluto

22. Quiz #1 What did JJ Thomson contribute to the model of the atom? • using experimental evidence to show elements • determining that electrons orbit in distinct levels • proving the the presence of a nucleus and the atoms are made of mostly empty space • proving that a difference in charge is present including quantized negative charge • none of these

23. Quiz #2 • Who did the following experiment? Measured the deflection of cathode rays across an electric potential. a. Democritus b. Dalton • J.J. Thomson • Rutherford e. Niels Bohr

24. Quiz #3 Which of the following elements is NOT present in the mixture? a. lithium b. hydrogen • Helium • sodium • more than one element is missing

25. Quiz #4 Which of the following elements has the smallest energy difference between electron energy levels? a. lithium b. hydrogen • Helium • Sodium

26. Quiz #5 You are trying build a scale model of an atom. If you decided that the electron should be the size of a basketball (25 cm) then how big should the proton be? • .25 cm pea • 2.5 cm golf ball • 25 m Planet Hollywood Ball • 250 m Ball one about the size of the RHHS campus • 2,500,000 m ball about ¼ of the Earth

27. Quiz #6 You are trying build a scale model of an atom. If you decided that the electron should the size of a water tower ball (4 m) then how far apart should the proton an electron be? • Proton in this class Electron on Football field (500 m) • Proton in this class Electron at RHS (5000 m) • Proton in this class Electron in downtown Dallas (32000 m) • Proton in this class Electron Washington D.C. (800,000 m) • Proton in this class Electron on the moon (400,000,000 m)