Nuclear Chemistry

1. Nuclear Chemistry The study of changes to the nucleus of the atom.

2. The Nucleus Comprised of protons and neutrons (nucleons). # protons = atomic number. # protons + neutrons = mass number

3. Isotope Review Isotope: Atoms of same element with different numbers of neutrons. • Have different levels of “abundance” in nature. • Some isotopes or “nuclides” of an element can be unstable, or “radioactive”. • Example of Carbon Isotopes Note: We will be talking about isotopes very specifically in this unit. We will not be using the average atomic mass you see on the Ref tables.

4. What is Radioactivity? • Radioactivity: the “decay” of the nucleus by emitting particles and/or energy in order to become more stable.

5. What Causes an Isotope to be Radioactive and Decay? Proton : Neutron ratio in nucleus

6. Neutron-Proton Ratios • Positive protons in the nucleus repel each other. • Neutrons play a key role stabilizing the nucleus.

7. Neutron-Proton Ratios For smaller nuclei (atomic # below 20) stable nuclei have a neutron-to-proton ratio close to 1:1.

8. Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus. There are no stable nuclei with an atomic number greater than 83.

9. Early Pioneers in Radioactivity Rutherford: Discoverer Alpha and Beta rays 1897 Roentgen: Discoverer of X-rays 1895 The Curies: Discoverers of Radium and Polonium 1900-1908 Becquerel: Discoverer of Radioactivity 1896 Marie Curie 3 parts 7 minutes each http://www.youtube.com/watch?v=Uaiq-eus-c0&safe=active http://www.youtube.com/watch?v=eDRk1gTvg30&safe=active http://www.youtube.com/watch?v=BIIC2KYoAEo&safe=active

10. RUTHERFORD DISCOVERS DIFFERENT TYPES OF RADIATION Ernest Rutherford discovered three types of radioactive emissions by using a magnetic field.

11. Reference Table O • Shows the symbols of some of the different particles used in nuclear chemistry. Top # = mass Bottom # = charge

12. Types ofRadioactive Decay http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/radioa7.swf Cloud Chamber: https://www.youtube.com/watch?v=chxv5G6UFl0

13. 238 92 234 90 4 2 4 2 He U Th He +  Alpha Decay An -particle is emitted (basically a helium nucleus)

14. Heaviest type of emission • Mass of 4 • Charge of +2

15. 131 53 131 54 0 −1 0 −1 0 −1 e I Xe e - +  or Beta Decay A - particle is emitted (a high energy electron)

16. Wait a tic • How does a nucleus give off an electron! • Neutron splits into a proton and electron. n → p+ + e- • Proton stays behind and electron shoots out of nucleus.

17. 0 +1 0 +1 11 6 11 5 e C B e +  Positron Emission A positron is emitted (a particle that has the same mass as but opposite charge than an electron)

18. Positrons are a type of “antimatter”. • Quickly destroyed as soon as they come in contact with an electron.

19. 0 0  Gamma Emission • High-energy radiation that almost always accompanies the loss of a nuclear particle. • It is NOT a particle, it is pure energy. • No mass or charge. • Not affected by a magnetic field.

20. 0 −1 1 1 1 0 p e n +  Electron Capture An electron close to nucleus get “captured’. It combines with a proton to make a neutron.

21. Penetrating Power • Penetrating Power: how far radiation can travel through material. • Protection requires different degrees of shielding. Alpha – paper or skin Beta – aluminum foil Gamma – thick lead

22. Ionizing Ability • Ionizing Ability: how well radiation strips electrons from other atoms and molecules creating ions. • Can cause mutations, and cell destruction • Alpha - Highest • Beta - Middle • Gamma – Low

23. Damage to Cells • Because of high ionizing ability, Alpha and Beta cause most damage inside the human body. • Gamma rays are less ionizing but protection against gammas requires thicker shielding.

24. Measuring Radioactivity • One can use a device like a Geiger counter to measure the amount of activity present in a radioactive sample.

25. Natural Transmutation (Decay) • “Spontaneous” transmutation of a radioisotope into another element. • Doesn’t require the input of outside energy. • Occurs at a specific rate that we can measure. (Half Life)

26. Radioactive Series • Decay Series: very large radioactive nuclei undergo a “series” of decays until they form a stable nuclide (often a nuclide of lead).

27. Artificial Transmutation • The change of one element to another artificially by bombarding it with other particles. • These equations always have 2 reactants on the left (as opposed to natural decay) Artificial Transmutation Natural Transmutation

28. How do We Bombard Nuclei? Particle Accelerators: Speed up charged particles in a magnetic field to collide with nuclei Neutrons and gamma radiation can’t be accelerated as they have no charge!

29. Transuranium Elements: • Elements “beyond” uranium (largest natural element) • Atomic numbers greater than 92 • Artificially created through nuclear bombardment

30. The Science Show: 4 New Elements added to Periodic Table https://www.youtube.com/watch?v=h9bzQIsQMAI Video: Islands of Stability http://www.youtube.com/watch?v=woPx-Ex7H8A&safe=active

31. Typical Particle Accelerator Enormous, with circular tracks with radii that are miles long.

32. Brookhaven Accelerator

33. Balancing Nuclear Equations • Mass and charge are “conserved” • Balance so that the mass (top #’s) and charge (bottom #’s) equal each other.

34. Typical Test Questions

36. Half Life • Amount of time for half a radioactive sample to decay. • Length of a half life cannot be changed. • Ranges from milliseconds to billions of years. (See Table N) • Radioactivity decreases with time.

37. Radioactive Dating • Rate of decay is constant over time. • Measure amount of radioisotope remaining in sample to determine age. • C-14 is used to date organic material up to 60,000 years old. • U-238 is used to date extremely old geological formations • Carbon 14 Dating:n (2 minutes) • http://www.youtube.com/watch?v=31-P9pcPStg&safe=active

38. Reference Table N • Decay mode: type of particle emitted by natural decay • Half Life: length of time for “half” of the atoms in a sample to undergo natural decay.

39. Half Life Basic Formula: # Half Lives = Total Time Elapsed Time of One Half Life (t1/2 )

40. Half Life Problem • Ex: If 500 grams of I-131, t1/2 = 8 days, decays for 32 days, how much would remain? • 32 days = 4 half lives 8 days 500 g → 250 g→ 125 g → 62.5 g→ 31.25 grams

41. Half Life Problem • Ex: If 300g of a radioisotope decays to 37.5g in 120 days, what is the t1/2 ? 300 → 150 → 75 → 37.5g 3 half lives 3 half lives = 120 dayst1/2 = 40 days t1/2

42. Half Life Problem • Ex: What fraction of a sample of I-131 remains after 24 days of decay? t1/2 = 8 days 24 days = 3 half lives 8 days Start End 1 → ½ → ¼ → 1/8

43. Half Life Problem • Ex: If 60 g of N-16 remains in a sample. How many grams were present 28 seconds ago? t1/2 = 7 sec. 28 sec = 4 half lives AGO 7 sec We double going back in time 60 → 120 → 240 → 480 → 960 grams

44. After 32 days, 5 milligrams of an 80-milligram sample of a radioactive isotope remains unchanged. • What is the half-life of this element? (1) 8 days      (2) 2 days   (3) 16 days(4) 4 days

45. An original sample of K-40 has a mass of 25.00 grams. After 3.9 × 109years, 3.125 grams of the original sample remains unchanged. • What is the half-life of K-40? (1) 1.3 × 109 y     (2) 2.6 × 109 y     (3) 3.9 × 109 y(4) 1.2 × 1010 y

46. What is the half-life of sodium-25 if 1.00 gram of a 16.00-gram sample of sodium-25 remains unchanged after 237 seconds? (1) 47.4 s     (2) 79.0 s(3) 59.3 s     (4) 118 s

47. How many days are required for 200. grams of radon-222 to decay to 50.0 grams? (1) 1.91 days        (2) 3.82 days   (3) 7.64 days          (4) 11.5 days

48. Honors Half Life Equations • Radioisotopes each have a unique half-life. • Each will decay at a specific “rate” over time. • Use the rate constant “k” to denote a specific rate constant for an isotope in half-life problems. k = .693 t1/2

49. log N0 = k x t N 2.3 N0 = original quantity N = final quanity t = total time k = decay constant (.693) t1/2

50. Half Life Graph http://www.absorblearning.com/media/attachment.action?quick=185&att=3167 Use the graph to see how much time it takes for half the nuclei to decay