2. The Evolution of the Atomic Model Democritus to Rutherford

3. Exploring the Black Box  Imagine you’ve received a present that you are not allowed to open. You REALLY want to know what’s inside!  What might you do to figure out the contents?

4. Exploring the Black Box  In your group, try to figure out the contents and internal structure your present WITHOUT opening it  Note: Do NOT shake the box too violently as the contents may fall out!  As a group, be sure to answer the questions on the hand-out!

5. So What is a Black Box?  A black box refers to any system that has inner workings that cannot be directly observed because it is not possible or feasible  Can you think of any other black box examples?  An atom is an example of a black box!

6. The Atomic Model and the Nature of Science  When scientists wanted to determine the structure of an atom, they were not able to actually see inside of the atom  They had to rely on empirical evidence, indirect evidence, observation, and inference when developing their ideas-just as you did when determining the contents of your present!

7. The Atomic Model and the Nature of Science  Scientists developed a model to express their new ideas  The understanding of the structure of the atom has changed many times over the course of centuries As new evidence emerged, the ideas of the atomic model changed  Lecturing about the scientists of the atomic model isn’t quite as fun and interesting as understanding:  HOW was the atomic model developed?  WHY has the atomic model changed over the years?

8. What is Today’s Atomic Model? Dense, Positively Charged Nucleus Mostly Empty Space Negatively Charged Electron Cloud Most Probable Location of the Electrons Composed of Protons, Neutrons, and Electrons

9. Timeline of Development of Current Atomic Model 1913 450 BC Democritus proposed the idea of atomos. 1802 Beginning of Modern Atomic Theory 1897 Discovery of the Electron 1911 Discovery of the Nucleus The Idea of Energy Levels for Electrons was Proposed. 1930 Introduction of the wave mechanical model Discovery of the Proton Discovery of the Neutron

10. Democritus  The Greek philosopher Democritus began the search for a description of matter more than 2400 years ago  He described the material world as made up of tiny, indivisible, hard particles called atoms  “Atomos” – not to be cut  All atoms were made of the same material but were different shapes and sizes 400 BC

11. Democritus vs. The Philosophers  The eminent philosophers of the time, Aristotle and Plato, formulated the notion that there can be no ultimately “atomic” view of matter  This dominated Western ideas for many centuries  Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter

12. Then Along Came Dalton’s Model  The notion of an atomic theory of matter reemerged during the 17 th century as a result of the work of John Dalton  The English chemist performed a number of experiments that eventually led to the acceptance of the idea of atoms  His theory is comprised of 4 postulates and is commonly referred to as the billiard ball model

13. Dalton’s Postulate #1  Each element is composed of extremely small particles called atoms

14. Dalton’s Postulate #2  All atoms of a given element are identical H H

15. Dalton’s Postulate #2  But, the atoms of one element are different from the atoms of all other elements O H

16. Dalton’s Postulate #3 Atoms of one element cannot be changed into atoms of a different element by chemical reactions In other words, atoms are neither created nor destroyed in chemical reactions They are indestructible!

17. Dalton’s Postulate #4 Compounds are formed when atoms of more than one element combine A given compound always has the same relative number and kinds of atoms

18. The Discovery of Atomic Structure  The ancient Greeks were the first to postulate that matter was indivisible  Even Dalton did not have any direct evidence for the existence of atoms!  Later, scientists realized that the atom consisted of subatomic, charged particles  What landmark discoveries led to this conclusion?

19. Thomson’s Atomic Model  In 1897, the work of an English scientist J. J. Thomson provided the first hint that atoms were made up of even smaller particles

20. Thomson’s Experiment  Thomson studied electrical discharge through a glass tube pumped almost empty of neutral air  When a high voltage was applied to the electrodes, negatively-charged radiation, called cathode rays, would travel from the negative electrode to the positive electrode  The path of the cathode rays could be altered by the presence of a magnetic field

21. Cathode Ray Tube

22. 21 Cathode-Ray Tubes – Ever Seen One? http://www.howstuffworks.com/tv4.htm

23. Thomson Model  Thomson observed that cathode rays behaved identically regardless of the identity of the metal used as cathode material  He described cathode rays as streams of negatively-charged particles which came from within the atom

24. 23 Thomson’s Conclusions  “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 “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."

25. Cathode rays (electrons) are tiny, negatively charged “corpuscles” Since atoms are neutral, Thomson inferred that these electrons must be surrounded by a “cloud” of positive charge

26. J J Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model Never had plum pudding? Think of a blueberry muffin

27. 26 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

28. 27 More Pieces of the Atom Ernest Rutherford (1871-1937)  Nuclear physicist from New Zealand  Thomson’s student  Taught at Cambridge University in Great Britain

29. Ernest Rutherford’s Atomic Model (1871- 1937)  Electrons embedded in a positive pudding Where exactly are those electrons? Thomson’s Theory: “Plum Pudding”  Shoot something at them to see where they are! Rutherford’s idea:

30. Rutherford Has an Idea… What if I shoot alpha radiation at gold atoms in gold foil?

31. 30 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

32. Rutherford’s Prediction Based on Thomson’s Model  The alpha particles to pass through without changing direction very much  This is because the positive charges were spread out evenly  Alone, they were not enough to stop the alpha particles

33. What Rutherford Expected

34. He thought the mass was evenly distributed in the atom

35. The Results Rutherford Actually Got…

36. Let’s See for Ourselves! +

37. Rutherford’s Conclusion about the Atom +  Atom is mostly empty space  It has a small, dense, positive center called the nucleus  Alpha particles are deflected by the nucleus if they get close enough

38. 37 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

39. 38 Properties of Subatomic Particles Property Particle Mass (g)Relative Charge Location in Atom Electron9.110 x 10 -28 - 1Outside nucleus Proton1.673 x 10 -24 + 1Inside nucleus Neutron1.675 x 10 -24 0Inside nucleus

40. The Atomic Model Evolves…  Rutherford’s model became known as the “planetary model”  The “sun” was the positively- charged dense nucleus and the negatively-charged electrons were the “planets”

41. The Planetary Model is Doomed!  By all the rules of classical physics, an electron in orbit around an atomic nucleus would be very unstable  This is because the orbiting electrons would give off energy as it is continually accelerating in a curved path  Like a satellite orbiting Earth  Resulting loss of energy implies that the electron would eventually crash into the nucleus and the atom would collapse!

42. Why Doesn’t the Model Work?

43. WAIT…What?! Electron crashes into the nucleus!? Since this does not happen, the Rutherford model could not be accepted!

44. Along Comes Neils Bohr…  Bohr worked with Rutherford and made the assumption that the charged electron simply does not fall into the nucleus  He stated that the nucleus is at the center of the atom  Electrons can only exist at specific energy levels called orbits  Each new ‘orbit’ is a new layer in the atom  This was the only way electrons could exist for a long time without giving off energy

45. Shortcomings of the Bohr Model  Bohr’s model was too simple  Worked well with only hydrogen because H only has one electron  Bohr also avoided the problem of why the negatively-charged electron would not just fall into the positively charged nucleus, by simply assuming it does not happen  Furthermore, there is a problem with describing an electron merely as a small particle moving in circular orbits around the nucleus So there is more to the atomic puzzle…

46. 45 The Position of Electrons Can’t be Predicted Precisely 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

47. 46 Electrons Move in “Probability Clouds”, not Circular Orbits  The exact path of an electron can’t be predicted  If we know the electron is somewhere in the atom, it’s velocity is uncertain by ~7,300 km/s (~ 16 million mph)!

48. Introducing the Quantum Mechanical Model!  Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions of space (orbitals)  NOT CIRCULAR ORBITS

49. 48 In Other Words… 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.”