History of the Atom

1. History of the Atom

2. History of the atom • Not the history of atom, but the idea of the atom. • Original idea Ancient Greece (400 B.C.) • Democritus - Greek philosopher.

3. History of Atom • Smallest possible piece? • Atomos - not to be cut • Looked at beach • Made of sand • Cut sand - smaller sand

4. Another Greek • Aristotle - Famous philosopher • All substances are made of 4 elements • Fire - Hot • Air - light • Earth - cool, heavy • Water - wet • Blend these in different proportions to get all substances

5. Who Was Right? • Did not experiment. • Greeks settled disagreements by argument. • Aristotle was a better debater - He won. • His ideas carried through middle ages. • Alchemists tried to change lead to gold.

6. Who’s Next? • 1803 - John Dalton- England. • Teacher- summarized results of his experiments and those of others. • Elements substances that can’t be broken down • In Dalton’s Atomic Theory • Combined idea of elements with that of atoms.

7. Dalton’s Atomic Theory • All matter is made of tiny indivisible particles called atoms. • Atoms cannot be destroyed. In chemical reactions, the atoms rearrange, but do not break apart. • All atoms of the same element have the same properties and identical masses. • Atoms of different elements have different properties and masses. • When atoms of one element combine with the atoms of another element, they always combine in the same proportional ratio.

8. Dalton’s Theories Supported Law of Definite Proportion (Chemical Composition) Law of Conservation of Mass Law of Multiple Proportions

9. Law of Conservation of Mass Mass is neither created or destroyed in a chemical reaction.

10. Parts of Atoms • J. J. Thomson - English physicist. 1897 • Made a piece of equipment called a cathode ray tube. • It is a vacuum tube - all the air has been pumped out. • A limited amount of other gases are put in

11. Voltage source Thomson’s Experiment - + Metal Disks

12. Voltage source Thomson’s Experiment - + • Passing an electric current makes a beam appear to move from the negative to the positive end

13. Voltage source Thomson’s Experiment + - • By adding an electric field

14. Voltage source Thomson’s Experiment + - • By adding an electric field he found that the moving pieces were negative • By adding an electric field

15. Thomson’s Experiment • Used many different metals and gases • Beam was always the same • Ray was deflected if a magnetic or electrical field were brought near • Found that cathode ray would bend toward the positive pole  The cathode rays were negatively charged; were called electrons

16. Thomsom’s Model • Discovered the electron. • Model shows negatively charged electrons embedded into an atom filled with positively charged material • Said the atom was like plum pudding.

17. Atomizer + - Oil Microscope Metal Plates Millikan’s Experiment - 1909

18. Atomizer Oil droplets + - Oil Microscope Millikan’s Experiment

19. Millikan’s Experiment X-rays X-rays give some drops a charge by knocking offelectrons

20. + - Millikan’s Experiment

21. Millikan’s Experiment - - + + They put an electric charge on the plates

22. Millikan’s Experiment - - + + Some drops would hover

23. + Millikan’s Experiment - - - - - - - Some drops would hover + + + + + + +

24. Millikan’s Experiment - - + + From the mass of the drop and the charge on the plates, he calculated the charge on an electron

25. Rutherford’s Experiment • Ernest Rutherford English physicist. (1911) • Believed the plum pudding model of the atom was correct. • Wanted to see how big they are. • Used radioactivity. • Alpha particles - positively charged pieces given off by uranium. • Shot them at gold foil which can be made a few atoms thick.

26. Rutherford’s experiment • When the alpha particles hit a florescent screen, it glows.

27. Flourescent Screen Lead block Uranium Gold Foil

28. He Expected • The alpha particles to pass through without changing direction very much. • Because… • The negative charges were spread out evenly. Alone they were not enough to stop the alpha particles.

29. What he expected

30. Because

31. Because, he thought the mass was evenly distributed in the atom

32. Because, he thought the mass was evenly distributed in the atom

33. What he got

34. + How he explained it • Atom is mostly empty. • Small dense, positive piece at center. • Alpha particles are deflected by it if they get close enough.

35. +

36. Density and the Atom • Since most of the particles went through, it was mostly empty. • Because the pieces turned so much, the positive pieces were heavy. • Small volume, big mass, big density. • This small dense positive area is the nucleus.

37. Niels Bohr - 1922 Applied quantum theory to Rutherford's atomic structure by assuming that electrons travel in stationary orbits around the nucleus. Often referred to as the “Planetary Model” This led to the calculation of possible energy levels for these orbits and the postulation that the emission of light occurs when an electron moves into a lower energy orbit.

38. Louis de Broglie - 1923 Thomson’s experiments demonstrated that electrons act like particles that have mass. de Broglie pointed out that the behavior of electrons was similar to the behavior of waves. (like light) Frequencies could correspond to the specific energy levels associated with the Bohr model.

39. Modern View • The atom is mostly empty space. • Two regions. • Nucleus- protons and neutrons. • Electron cloud- region where electrons are located in areas of high probability called orbitals

40. Other pieces • Proton - positively charged pieces 1840 times heavier than the electron. (Discovered in 1914 by Henry Moseley) • Neutron - no charge but the same mass as a proton. (Discovered by James Chadwick in 1932) • Where are the pieces?