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 the 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”. • 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.

9. Stable Nuclei • There are no stable nuclei with an atomic number greater than 83.

10. Types ofRadioactive Decay http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/radioa7.swf

11. 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

12. Ernest Rutherford discovered three types of radioactive emissions by using a magnetic field.

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

14. 238 92 234 90 4 2 4 2 He U Th He +  Alpha Decay An -particle is emitted (basically a helium nucleus)

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

16. 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)

17. 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.

18. 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)

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

20. 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.

21. Don’t make me mad… Ms. Nelson on a bad day…

22. 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.

23. 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

24. 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

25. 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.

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

27. 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)

28. 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

29. 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!

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

31. Brookhaven Accelerator

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

33. Typical Test Questions

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

36. Transuranium Elements: • Elements “beyond” uranium (largest natural element) • Atomic numbers greater than 92 • Artificially created through nuclear bombardment Video: Islands of Stability (13 minutes) http://www.youtube.com/watch?v=woPx-Ex7H8A&safe=active

37. 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.

38. 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

39. 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.

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

41. 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

42. 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

43. 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

44. 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

45. 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

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

47. Half Life Graph http://www.colorado.edu/physics/2000/isotopes/radioactive_decay3.html Use the graph to see how much time it takes for half the nuclei to decay

50. Energy in Nuclear Reactions • Nuclear reactions yield more energy than chemical reactions • When changes happen to the nucleus, some matter is converted to energy. • Einstein’s famous equation, E = mc2, allows us to calculate this energy.