1. Teaching Resources HST 2008

2. A brief History of our thinking The Greek thinker Empedocles first classified the fundamental elements as fire, air, earth, and water, although our particular diagram reflects Aristotle's classification. Did you know? The ancient Chinese believed that the five basic components (in Pinyin, Wu Xing) of the physical universe were earth, wood, metal, fire, and water. And in India, the Samkhya-karikas by Ishvarakrsna (c. 3rd century AD) proclaims the five gross elements to be space, air, fire, water, and earth.

3. People have long asked, "What is the world made of?" and "What holds it together?" As far back as in the days of Aristotle, it was thought that things were made up of four types of fundamental elements. The word "fundamental" is key here. By fundamental building blocks we mean objects that are simple and structureless -- not made of anything smaller.

4. Based on scientific observations, our thinking has variously changed in the recent past. The story begins with John Dalton...... Dalton suggested that … … everything is made up of very tiny particles. He named these smallest possible piece of an element ‘an atom’ – which in Greek means unbreakable. Further he suggested that…. Atoms are different for different elements. Imagine atoms to be solid like billiard balls.

5. Oxygen AtomHydrogen Atom Gold Atom

6. From our scientific observations in chemical reactions, x-ray diffraction etc, we know today that this view was not entirely correct. Something was missing.. However, Dalton’s ideas became the basis for our modern quest for the real constituents of matter (and antimatter!) Possible web link to a CERN site with pictures, simulations or videos?

7. Gold Atom Proposed after discovery of “cathode rays” when a gas is ionized by a high voltage. Neutral atoms contain smaller particles, called electrons. Electrons exist in a “sea of positive charge” like plums in a pudding.

8. Shot high energy a particles at a thin gold foil Observed the pattern of scattered particles Found some scattered by a larger angle than predicted by the Thomson model

9. Positive charge is concentrated in the nucleus (only way to produce large scattering angles observed) Coulomb force causes electrons to orbit the positive nucleus Planetary model

10. Accelerating electrons should radiate energy according to Maxwell’s Electromagnetic theory Radiating electrons are losing energy so they will spiral in towards the nucleus as they lose energy No atom would be stable -- all would radiate away their energy Hydrogen Atom

11. Predicted Hydrogen Emission Spectrum as atoms radiate energy

12. Hydrogen electrons may only exist at certain distances from the nucleus (energy levels) If they stay in the same energy level, they are stable (don’t radiate) If they move from one level to another, they radiate to produce the discrete spectrum observed

13. Ionized gas atoms are injected into a mass spectrometer All atoms have the same charge and the velocity selector guarantees that they had the same velocity Different radii are observed in the B-field deflection Only way for this is to have atoms with same charge and different mass. (R = mv/qB) There must be a neutral particle in the nucleus with significant mass = neutron Atoms with same charge (protons) and different mass (neutrons) are called Isotopes

14. Helium Atom electron nucleus

15. Specific Nucleus = nuclide Nuclear particles (protons and neutrons) = nucleons Identically charged nuclei with different mass = isotopes Generic nucleus symbol = Number of nucleons = A = mass number Number of protons = Z = charge (atomic) number

16. Clearly, if the nucleus contains a number of protons, the Coulomb force would predict that the protons should repel each other There must some force that is stronger than the Coulomb force to hold them together = Strong Nuclear Force Strong Nuclear Force also keeps neutrons in check in the nucleus Acts like a spring between nucleons FEFE FEFE

17. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

18. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

19. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

20. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

21. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

22. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

23. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

24. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

25. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

26. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

27. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

28. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

29. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

30. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

31. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle

32. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle charge = +2e E = KE

33. We can use the results from the scattering of positively charged particles to determine the nuclear radius charge = +Ze Alpha Particle KE Alpha Particle PE  Particle charge = +2e E = PE = R

34. We can use the results from the scattering of positively charged particles to determine the nuclear radius Final E = PE = Initial E = (KE) Conservation of Energy: (KE) = Solve for R =

35. We can use the results from the scattering of positively charged particles to determine the nuclear radius Solve for R = Adjust initial KE until the a particle is no longer scattered back. Last scattered KE gives estimate of R. Empirical Result: R = (1.2 x 10 -15 )A 1/3

36. Alpha particles emitted by an unstable nucleus are found to have limited possible amounts of kinetic energy Gamma rays (pure energy) emitted by an excited nucleus are found to produce discrete spectra These results suggest that nuclei have energy levels just like atoms do

37. The nature of the strong nuclear force is that it is effective over very small distances  Small nuclei are stable when the number of neutrons = number of protons. Z A - Z 10 20 3040 506070 80 20 40 60 80 10 0 12 0 = stable Segre Plot of Stable Nuclides

38. The nature of the strong nuclear force is that it is effective over very small distances  As Z increases, the number of neutrons necessary becomes larger than the number of protons. Z A - Z 10 20 3040 506070 80 20 40 60 80 10 0 12 0 = stable Segre Plot of Stable Nuclides

39. The nature of the strong nuclear force is that it is effective over very small distances  For large values of Z, the number of neutrons becomes very large. Z A - Z 10 20 3040 506070 80 20 40 60 80 10 0 12 0 = stable Segre Plot of Stable Nuclides

40. The nature of the strong nuclear force is that it is effective over very small distances For large nuclei, the Coulomb force wins out over the strong nuclear force Z A - Z 10 20 3040 506070 80 20 40 60 80 10 0 12 0 = stable Segre Plot of Stable Nuclides

41. The nature of the strong nuclear force is that it is effective over very small distances For large nuclei, the Coulomb force wins out over the strong nuclear force = range of nuclear force this proton can repel the proton on the opposite side of the nucleus this neutron is too far away to exert an attractive nuclear force on the nucleons on the opposite side of the nucleus

42. The nature of the strong nuclear force is that it is effective over very small distances In addition, as the nucleons become too tightly packed the nuclear force will cause them to repel each other. = range of nuclear force this proton can repel the proton on the opposite side of the nucleus this neutron is too far away to exert an attractive nuclear force on the proton on the opposite side of the nucleus

43. To achieve stability, the nucleus must either decrease its size by emitting clusters of nucleons (a particles)  particle emission

44. To achieve stability, the nucleus must either decrease its size by emitting clusters of nucleons (a particles)  particle emission Or, it must rearrange the nucleons to balance the nuclear force and Coulomb force (b particles)  particle emission neutron decays into proton and  particle