Chapter 5 The Atomic Nucleus

1. Lecture Presentation Chapter 5The Atomic Nucleus Bradley Sieve Northern Kentucky University Highland Heights, KY

2. 5.1 Radioactivity Results from Unstable Nuclei • Stable Nuclei • Due to the ratio of protons to neutrons • Unstable Nuclei • Contains an “off-balance” ratio of protons and neutrons • Transforms the nucleus to a more stable composition

3. 5.1 Radioactivity Results from Unstable Nuclei • Radioactive Materials • Material containing unstable nuclei • Radioactivity • Emitted high-energy particles and radiation from unstable nuclei • Radioactive Decay • Process of emitting radioactivity

4. 5.1 Radioactivity Results from Unstable Nuclei • Three types of radiation are emitted • Alpha (α) • Alpha particles carry a positive electric charge • Beta (β) • Beta particles carry a negative electric charge • Gamma (γ) • Gamma particles carry no electric charge

5. 5.1 Radioactivity Results from Unstable Nuclei

6. 5.1 Radioactivity Results from Unstable Nuclei • Alpha Radiation • Releases a stream of alpha particles • Alpha Particle • Contains two protons and two neutrons • The same as a helium nuclei • Low penetrating power due to large mass (2 a.m.u.) and double positive charge (+2)

7. 5.1 Radioactivity Results from Unstable Nuclei • Beta Radiation • Releases a stream of beta particles • Beta Particle • Is simply an electron ejected • Medium-range penetrating power with small mass and a single negative charge

8. 5.1 Radioactivity Results from Unstable Nuclei • Gamma Radiation • High-frequency electromagnetic radiation • Gamma Particle • Is pure energy • Highest penetrating power as there is no mass or charge with the particle

9. 5.1 Radioactivity Results from Unstable Nuclei

10. 5.1 Radioactivity Results from Unstable Nuclei

11. 5.2 Radioactivity Is a Natural Phenomenon • Radioactivity has always been present • Contained in soil, air, the earth’s core • Most radiation is natural background radiation • 81% natural sources • 15% medical and diagnostic sources • 4% consumer products

12. 5.2 Radioactivity Is a Natural Phenomenon

13. 5.2 Radioactivity Is a Natural Phenomenon • Radon-222 • Is a common source of radiation • Arises from uranium rocks • Can collect in basements to unsafe levels

14. 5.2 Radioactivity Is a Natural Phenomenon • Mutations • Alterations in genetic information contained in our cells • Normally harmless but may cause conditions such as many types of cancer • May be passed on to offspring if damage is in a person’s reproductive cells

15. 5.2 Radioactivity Is a Natural Phenomenon • Rems are units of radiation • Units measure the ability of radiation to cause harm • Lethal doses of radiation begin at 500 rems • Often measured as millirems

16. 5.2 Radioactivity Is a Natural Phenomenon • Radioactive Tracers • Allow molecules to be traced due to radiation • Generally act the same as nonradioactive molecules

17. 5.2 Radioactivity Is a Natural Phenomenon • Medical Imaging • Isotopes used for the diagnosis of internal disorders • Tracer path not influenced by radioactive properties

18. 5.2 Radioactivity Is a Natural Phenomenon

19. 5.3 Radioactivity Results from an Imbalance of Forces • Strong nuclear force • Very strong force between nucleons • Only effects very short distances • Repulsive electric forces effect over relatively long-ranges

20. 5.3 Radioactivity Results from an Imbalance of Forces • Protons far apart may experience stronger repulsive force than the attractive strong nuclear force

21. 5.3 Radioactivity Results from an Imbalance of Forces

22. Concept Check Two protons in the atomic nucleus repel each other, but they are also attracted to each other. Why?

23. Concept Check While two protons repel each other by the electric force, they also attract each other by the strong nuclear force. These forces act simultaneously.

24. 5.3 Radioactivity Results from an Imbalance of Forces • Neutrons act as the “nuclear cement” • Neutrons feel attraction but not repulsion because they do not have a charge

25. 5.3 Radioactivity Results from an Imbalance of Forces • Neutrons can decay to a proton and an electron • This will destabilize the nucleus by increasing the number of protons

26. 5.4 Radioactive Elements Transmute to Different Elements • Transmutation • Changing of one element to another uranium transmuting into thorium

27. 5.4 Radioactive Elements Transmute to Different Elements • Nuclear Equation • Mass numbers at the top balance • Atomic numbers at the bottom also balance

28. 5.4 Radioactive Elements Transmute to Different Elements • Radioactive particle notations

29. 5.4 Radioactive Elements Transmute to Different Elements • Changes in atomic number by each type of particle release Alpha: decrease by two Beta: increase by one

30. 5.4 Radioactive Elements Transmute to Different Elements U-238 decays to Pb-206 through a series of alpha and beta decays

31. 5.5 The Shorter the Half-Life, the Greater the Radioactivity • Half-life • Radioactive isotopes decay at different rates • Decay rate is measured as an element’s half-life • Half-life is defined as the time required for half of the radioactive atoms to decay

32. 5.5 The Shorter the Half-Life, the Greater the Radioactivity

33. 5.6 Isotopic Dating Measures the Ages of Materials • N-14 is converted to C-14 in the atmosphere • C-14 is incorporated into plant life through photosynthesis • Animals also ingest C-14 by eating plants

34. 5.6 Isotopic Dating Measures the Ages of Materials • C-14 decays back to C-12 over time • Plants replenish C-14 while growing to maintain a constant amount • Once plant dies, amount of C-14 decreases

35. 5.6 Isotopic Dating Measures the Ages of Materials • Carbon-14 Dating • Used to calculate the age of carbon-containing artifacts • Half of the C-14 decays in about 5730 years • Exhibits roughly a 15% error rate

36. 5.7 Nuclear Fission—The Splitting of Atomic Nuclei • Nuclear Fission • Splitting a large nucleus into smaller halves • Releases enormous amounts of energy

37. 5.7 Nuclear Fission—The Splitting of Atomic Nuclei • Chain Reaction • When a product of one reaction begins the next reaction • Neutrons propagate the chain reaction for U-235

38. 5.7 Nuclear Fission—The Splitting of Atomic Nuclei • Nuclear Fission Reactors • Produces electrical energy from nuclear reactions • 1 kg of U yields more energy than 30 freight-car loads of coal

39. 5.7 Nuclear Fission—The Splitting of Atomic Nuclei • Major components of a fission reactor • Nuclear fuel • U-238 mostly with about 3% U-235 • Control rods • Neutron-absorbing material • Moderator • Slows the velocity of the neutrons • Liquid • Transfers the heat from the reactor

40. 5.7 Nuclear Fission—The Splitting of Atomic Nuclei • Radioactive Waste • By-products of nuclear reactions • Half-lives range from short to thousands of years • Disposal of waste is problematic

41. 5.8 The Mass-Energy Relationship. E=mc2 • Albert Einstein • Postulated that mass and energy are two sides of the same coin E=mc2 E = energy m = mass c = speed of light

42. 5.8 The Mass-Energy Relationship. E=mc2 • Variation of average mass based on the nucleus that contains it • Nucleons lose mass in their transition from uranium to a smaller fragment

43. 5.9 Nuclear Fusion—The Combining of Atomic Nuclei • Nuclear Fusion • Combining two light nuclei to form a larger nucleus • Mass is lost during the fusion process • Lost mass is released as energy

44. 5.9 Nuclear Fusion—The Combining of Atomic Nuclei • Thermonuclear Fusion • Fusion brought about by high temperatures • Can be initiated by a fission reaction

45. 5.9 Nuclear Fusion—The Combining of Atomic Nuclei • Controlling fusion can be done using plasmas • Magnetic straitjacket for hot ionized gases • Currently no commercial fusion power plants in use