1. Nuclear Chemistry

2. Atomic Nucleus  Very small  Very dense ----1.6 x 10 14 g/cm 3 ----Ping-Pong ball of nuclear matter = 2.5 billion tons of mass  Large magnitude of energy in nucleus  Composed of neutrons and protons  Neutrons & protons made of quarks

3. Nuclear Vocabulary Nucleons: particles in the nucleus: – p + : proton – n 0 : neutron. Mass number (A): the number of p + + n 0. Atomic number (Z): the number of p +. Isotopes: same number of p + ; different numbers of n 0. Nuclide: unique atom: A Z X

4. Why Are Some Nuclei Unstable? Proton has high mass and high charge. Proton-proton repulsion is large. In nucleus protons are very close to each other. Strong nuclear forces: cohesive forces in nucleus. Neutrons involved with strong nuclear force. As more protons are added the proton-proton repulsion gets larger.

5. Neutron-to-Proton Ratio The heavier the nucleus, the more neutrons are required for stability. The belt of stability deviates from a 1:1 neutron to proton ratio for high atomic mass.

6. Radioactive Decay  Many nuclei are unstable (out of 2000 nuclides, 279 are stable).  Light nuclides stable when n  : p + = 1  More stability with even numbers of n  and p +  Super stable: magic numbers 2, 8, 20, 28, 50, 82, 126  Nuclides with 84 or more protons are unstable. Radioactivity: decomposition of a nucleus to form another nucleus plus particle(s). In nuclear equations, number of nucleons is conserved:  238 92 U  234 90 Th + 4 2 He

7. Packet Example Page 7  Which of the following nuclei are especially stable: helium-4, calcium-40, or technetium-98?  helium-4 and calcium-40, because 1:1 proton:neutron ratio and even numbers  technetium-98, NO! Few nuclei are stable with odd numbers of protons and neutrons, plus above Z = 84

8. Three Types of Radioactive Decay Three types: –  -Radiation: loss of 4 2 He from nucleus –  -Radiation: loss of an e - from nucleus –  -Radiation: loss of high-energy photon from nucleus Write all particles with their atomic and mass numbers: 4 2 He and 4 2  represent  -radiation

9. Alpha-particle Production   particle is a helium nucleus: 4 2 He  Very common decay for heavy radioactive nuclides  Decaying nucleus changes sheds  particle, changes mass 238 92 U  234 90 Th + 4 2 He

10. Practice Problems p 3  1. What product is formed when radium-226 undergoes alpha decay?  2. What element undergoes alpha decay to form lead-208?

11. Beta-radiation  Neutron splits into p + and e -  (Electron is created by release of energy)  High-speed electron is  particle   -radiation is loss of electron from nucleus and change of a neutron to a proton  Mass of decaying nucleus remains constant 1 0 n  1 1 p + + 0 -1 e -

12. Gamma Ray Production   -ray is high energy photon  Not a particle!  Part of EM spectrum  Accompanies some nuclear decays  Release of  -rays allows relaxation of excited nucleus to its ground state

14. Spontaneous Fission  Another decay process  Spontaneous fission: splitting of heavy nuclide into 2 lighter nuclides with similar mass numbers  Mass number changes as with  -decay  Occurs at very slow rate for most nuclides.

15. Positron Emmision  Occurs for nuclides whose neutron/proton ratio if too small  Positron: particle with mass of electron but opposite charge  Positron emission changes a proton to a neutron, causing a higher neutron/proton ratio than original atom. 1 0 p +  1 0 n + 0 1 e +

16. Positron Annihilation  Positron is the antiparticle of the electron.  Collision of electron and positron changes matter to EM gamma radiation. 0 -1 e - + 0 1 e +  2 0 0 

17. Electron Capture  Inner-orbital electron captured by nucleus  Proton captures electron and forms neutron  Occurs very slowly  Gamma rays are produced 1 1 p + + 0 -1 e -  1 0 n

18. Nuclear Stability At Bi (the belt of stability ends and all nuclei are unstable. Nuclei above belt have  - emission. Nuclei below belt have  + - emission or electron capture. Nuclei with atomic numbers greater than 83 usually have  -emission.

20. Packet Examples Pages 4-5  Write nuclear equations for the following processes:  a. Mercury-201 undergoes electron capture  b. Thorium-231 decays to form protactinium-231

21. Packet Examples Page 6  Predict the mode of decay of  carbon-14  xenon-118

22. Short-Hand Notation Nuclear transmutations can occur using high velocity  -particles:  14 N + 4   17 O + 1 p. The above reaction is written in short- hand notation: 14 N( ,p) 17 O.

23. Example Packet Page 7  Write the balanced equation for the process summarized as 27 Al(n,  ) 24 Na

24. Patterns of Nuclear Stability A nucleus usually undergoes more than one transition on its path to stability. Radioactive series: series of nuclear reactions that accompany path to stability. Daughter nuclei: nuclei resulting from radioactive decay.

25. Radioactive Series For 238 U, first decay is to 234 Th (  -decay). 234 Th undergoes  -emission to 234 Pa and 234 U. 234 U undergoes  -decay (several times) to 230 Th, 226 Ra, 222 Rn, 218 Po, and 214 Pb. 214 Pb undergoes  -emission (twice) via 214 Bi to 214 Po which undergoes  -decay to 210 Pb. The 210 Pb undergoes  - emission to 210 Bi and 210 Po which decays (  ) to the stable 206 Pb.

26. Nuclear Transmutations Nuclear transmutations: change of one element into another by collisions between nuclei. First observed by Rutherford in 1919 (N to O) Irene & Frederic Joliot-Curie in 1933 (Al to P) Can occur using high velocity  -particles: 14 N + 4   17 O + 1 p To overcome electrostatic forces, charged particles need to be accelerated in particle accelerators before they react.

27. Kinetics of Radioactive Decay  Rate law for radioactive decay is first-order.  ln(N/N 0 ) = -kt  Half-life (t ½ ): time required for the number of nuclides to reach half the original value.  Remember: t ½ = 0.693/k k = decay constant

28. Half-lives Each isotope has a characteristic half-life. Half-lives not affected by temperature, pressure or chemical composition. Half-lives of natural radioisotopes longer than half-lives of synthetic radioisotopes.  Half-lives range from fractions of a second to millions of years.

29. Units of Radioactivity  Activity: rate of decay (disintegrations per unit of time in seconds: dps)  SI unit of radioactivity = Becquerel (Bq)  1 Bq = 1 dps  Older Curie (Ci) still widely used  1 Ci = 3.7 x 10 10 dps

30. Radioactive Dating Naturally occurring radioisotopes can be used to determine how old a sample is. Carbon-14 is used to determine ages of organic compounds. We assume the ratio of 12 C to 14 C has been constant over time. Object must be less than 50,000 years old.  Shroud of Turin dated 1260-1390 A.D.

31. Examples Packet Top Page 9  The half-life of cobalt-60 is 5.3 years. How much of a 1.000 mg sample of cobalt-60 is left after a 15.9 year period?  Half-life (t ½ ): time required for the number of nuclides to reach half the original value.

32. Packet Example Middle Page 9  Carbon-11, used in medical imaging, has a half-life of 20.4 min. The carbon-11 nuclides are formed, then incorporated into a desired compound. The resulting sample is injected into a patient and the medical image is obtained. The entire process takes five half-lives. What percentage of the original carbon-11 remains at this time? Assume you start with 1.00 g carbon-11.

33. Packet Example 1 Page 11  A rock contains 0.257 mg of lead-206 for every milligram of uranium-238. The half-life for the decay of uranium- 238 to lead-206 is 4.5 x 10 9 yr. How old is the rock?  Remember: t ½ = 0.693/k  Remember: t = -1/k ln N/N 0

34. Packet Example 2 Page 11  A wooden object from an archeological site is subjected to carbon dating. The activity of the sample due to carbon-14 is measured to be 11.6 dps. The activity of a carbon sample of equal mass from fresh wood is 15.2 dps. The half-life of carbon-14 is 5715 yr. What is the age of the archeological sample?  Remember: t ½ = 0.693/k

35. Packet Example 3 Page 11  A sample to be used for medical imaging is labeled with fluorine-18, which has a half-life of 110 minutes. What percentage of the original sample remains after 300 minutes?

36. Detection of Radiation Matter is ionized by radiation. Geiger-Muller counter (Geiger counter) determines the amount of ionization of Ar by radioactive particles. Scintillation counter: measures flashes of light given off as substance is struck by radiation.

37. Geiger-Muller Counter

38. Medical Applications  Radiotracers: radioactive nuclides that can be put into organisms and whose pathways can be traced.  Sensitive and noninvasive  Iodine-131 diagnoses and treats illnesses of thyroid  Thallium-201 and technetium-99m assess damage to heart

39. Energy Changes in Nuclear Reactions Einstein showed that mass and energy are proportional: If a system loses mass it loses energy; if a system gains mass it gains energy. Since c 2 is a large number (8.99  10 16 m 2 /s 2 ) small changes in mass cause large changes in energy. Mass and energy changed in nuclear reactions are much greater than chemical reactions.

40. Energy as Matter  Energy is a form of matter!  Consider 238 92 U  234 90 Th + 4 2 He  For 1 mol the masses are 238.0003 g  233.9942 g + 4.0015 g.  Change in mass during reaction is 233.9942g+ 4.0015 g - 238.0003 g = -0.0046 g  The process is exothermic because the system has lost mass.

41. Mass Defect Mass of a nucleus is less than the mass of its nucleons! Mass defect: difference in mass between nucleus and component nucleons. Binding energy: energy needed to separate a nucleus into its nucleons.  Energy change =  E = ∆mc 2

42. Example Packet Top Page 14  How much energy is lost or gained when a mole of cobalt-60 undergoes beta decay to form nickel-60? The mass of cobalt-60 is 59.933819 amu and that of nickel-60 is 59.930788 amu.  Use  E = ∆mc 2

43. Example Packet Bottom Page 14  Positron emission from carbon-11 to form boron-11 occurs with the release of 2.87 x 10 11 joule per mol of carbon-11. What is the mass change per mole of carbon-11 in this nuclear reaction?  Use  E = ∆mc 2

44. Binding Energy Larger binding energy = more stable nucleus. Average binding energy per nucleon increases to a maximum at mass number 50 - 60, and decreases afterwards.  Most stable nucleus is iron-56.

45. Fission and Fusion  Energy is released when a process goes from less stable to more stable state.  Fusion: combining two light nuclei to form a heavier, more stable one.  Fission: splitting a heavy nucleus into two nuclei with smaller mass numbers.  Both processes energy changes millions of times larger than those from chemical reactions!

47. Nuclear Fission  Discovered in late 1930’s.  Fission of uranium-235 gives 26 million times more energy than combustion of CH 4.

48. Nuclear Chain Reactions For every 235 U fission 2.4 neutrons are produced. Each neutron produced can cause the fission of another 235 U nucleus. Number of fissions and energy increase rapidly. Eventually, a chain reaction forms. Without controls, an explosion results.

50. Critical Mass Each neutron can cause another fission. Minimum mass of fissionable material is required for a chain reaction (or neutrons escape before they cause another fission). When enough material is present for a chain reaction, we have critical mass. Subcritical mass: neutrons escape and no chain reaction occurs. Supercritical mass: any mass over critical

52. Nuclear Bomb Two subcritical wedges of 235 U separated by a gun barrel. Explosives used to bring two subcritical masses together to form one supercritical mass = nuclear explosion. Manhattan Project during WWII: Hiroshima and Nagasaki in 1945.

53. Nuclear Reactors  Controlled fission used to produce electricity. Use a subcritical mass of 235 U (enrich 238 U with about 3% 235 U). Enriched 235 UO 2 pellets are encased in Zr or stainless steel rods to absorb neutrons. Heat produced in the reactor core is removed cooling fluid to a steam generator. Steam drives an electric generator.

55. Future of Nuclear Reactors? Cons of Nuclear Reactors Pros of Nuclear Reactors  Storage of used fuel rods  Cost to build and maintain new reactors  Safety: Three-Mile Island and Chernobyl  Produces no greenhouse gases  Lower volume of waste products than fossil fuels  Almost unlimited supply of fuel  Excellent safety record

56. Nuclear Fusion Light nuclei can fuse to form heavier nuclei. Most reactions in the Sun are fusion. Fusion products are not usually radioactive, so fusion is a good energy source. Hydrogen needed for reaction can easily be supplied by seawater. However, high energies are needed to overcome repulsion between nuclei before reaction can occur.

57. Effects of Radiation  Energy transferred to cells can break chemical bonds and wreak havoc with sell systems.  Radiation damage can be subtle-feel effects years later.  Somatic damage: damage to organism = sickness of death  Genetic damage: produces malfunction in offspring

58. Biological Effects Depend on…  Energy of the radiation: dose measured in rads (radiation absorbed dose)  Penetrating ability of radiation:  <  <   Ionizing ability of radiation: extraction of electrons from biomolecules very damaging  Chemical property of radiation source: affects residence time in organism