Nuclear Chemistry

1. Nuclear Chemistry

2. Facts About the Nucleus • Very small volume compared to volume of atom • Essentially entire mass of atom • Very dense

3. Facts About the Nucleus (continued) • Composed of protons and neutrons that are tightly held together • Nucleons

4. Facts About the Nucleus (continued) • The nucleus of a specific isotope is called a nuclide • less than 10% of the known nuclides are nonradioactive, most are radioactive (radionuclides)

5. Nuclides • Each nuclide is identified by a symbol where: X = element symbol A = mass number Z = atomic number

6. Nuclear Reactionswith respect to other changes Energy drives all reactions, physical, chemical, biological, and nuclear. Physical reactions change states of material among solids, liquids, gases, solutions. Molecules of substances remain the same. Chemical reactions change the molecules of substances, but identities of elements remain the same. Biological reactions are combinations of chemical and physical reactions. Nuclear reactions change the atomic nuclei and thus the identities of nuclides. They are accomplished by bombardment using subatomic particles or photons.

7. Nuclear Reactionschanging the hearts of atoms Nuclear reactions, usually induced by subatomic particles a, change the energy states or number of nucleons of nuclides. b • After bombarded by a, the nuclide A emits a subatomic particle b, and changes into B. • a + A B + b • or written as A (a,b) B A (a,b) B a B A

8. Nuclear Reactions • Radioactive decay – a process by which the nucleus of a nuclide emits radioactive particles • Artificial Nuclear Transformation – the changing of one element into another by bombarding it with a nuclide • Nuclear Fission – the process of using a neutron to split a heavy nucleus into two smaller nuclei • Nuclear Fusion – the process of combining two light nuclei

9. Subatomic Particles for and from Nuclear Reactions Subatomic particles used to bombard or emitted in nuclear reactions:

10. Nuclear equations rules • Sum of reactant mass numbers = sum of product mass numbers • Sum of reactant atomic numbers = sum of product atomic numbers • “emitted” particles are on product side • “bombarding” or “captured” particles are on reactant side

11. Radiation • Radiation comes from the nucleus of an atom. • Unstable nucleus emits a particle or energy alpha  beta  gamma

12. alpha decay • an  particle contains 2 protons and 2 neutrons • helium nucleus

13. Alpha decay (continued)

14. Beta decay • a  particle is like an electron • moving much faster • found in the nucleus • in beta decay a neutron changes into a proton

15. Beta decay (continued)

16. gamma emission • Gamma () rays are high energy photons • Gamma emission occurs when the nucleus rearranges • No loss of particles from the nucleus

17. gamma emission (continued) • No change in the composition of the nucleus • Same atomic number and mass number • Generally occurs whenever the nucleus undergoes some other type of decay

18. positron emission • positron has a charge of +1 and negligible mass • anti-electron • positrons appear to result from a proton changing into a neutron

19. Positron emission (continued)

20. electron capture • occurs when an inner orbital electron is pulled into the nucleus • no particle emission, but atom changes • same result as positron emission

21. Electron capture (continued)

22. Artificial Nuclear Transformation • Nuclear transformation involves changing one element into another by bombarding it with small nuclei, protons or neutrons • reaction done in a particle accelerator

23. Artificial Nuclear Transformation (continued) • man-made transuranium elements

24. Other Nuclear Changes • a few nuclei are so unstable, that if their nucleus is hit just right by a neutron, the large nucleus splits into two smaller nuclei - this is called fission

25. Fission

26. Other Nuclear Changes (continued) • small nuclei can be accelerated to such a degree that they overcome their charge repulsion and are smashed together to make a larger nucleus - this is called fusion

27. Fusion

28. Other Nuclear Changes (continued) • both fission and fusion release enormous amounts of energy

29. Learning Check NR1 Write the nuclear equation for the beta emitter Co-60.

30. Solution NR1 Write the nuclear equation for the Beta emitter Co-60. 60Co 60Ni + 0 e 27 28 -1

31. Learning Check NR2 What radioactive isotope is produced in the following bombardment of boron? 10B + 4He ? + 1n 5 2 0

32. Solution NR2 What radioactive isotope is produced in the following bombardment of boron? 10B + 4He 13N + 1n 5 2 7 0 nitrogen radioisotope

33. Day 2 – Radioactivity Effects and Applications

34. Detecting Radioactivity • To detect something, you need to identify something it does • radioactive rays cause air to become ionized

35. Detection (continued) • Geiger-Müller Counter works by counting electrons generated when Ar gas atoms are ionized by radioactive rays

36. Detecting Radioactivity (continued) • radioactive rays cause certain chemicals to give off a flash of light when they strike the chemical • a scintillation counter is able to count the number of flashes per minute

37. Scintillation Counters Photons cause the emission of a short flash in the Na(Tl)I crystal.The flashes cause the photo-cathode to emit electrons.

38. Detecting Radioactivity (continued) • radioactive rays cause chemical changes in some materials • Photographic film is able to “record” its interactions with radioactive particles

39. Photographic Emulsions and Films Sensitized silver bromide grains of emulsion develope into blackened grains. Plates and films are 2-D detectors. Roentegen used photographic plates to record X-ray image. Photographic plates helped Beckerel to discover radioactivity. Films are routinely used to record X-ray images in medicine but lately digital images are replacing films. Stacks of films record 3-dimensional tracks of particles. Photographic plates and films are routinely used to record images made by electrons.

40. Half-Life • Not all radionuclides in a sample decay at once (random process) • The length of time it takes one-half the radionuclides to decay is called the half-life

41. Half-life (continued) • Even though the number of radionuclides changes, the length of time it takes for half of them to decay does not • the half-life of a radionuclide is constant

42. Half-life (continued) • Each radionuclide has its own, unique half-life • The radionuclide with the shortest half-life will have the greater number of decays per minute (For samples of equal numbers of radioactive atoms)

43. Half-Life of a Radioisotope The time for the radiation level to fall (decay) to one-half its initial value decay curve 8 mg 4 mg 2 mg 1 mg initial 1 half-life 2 3

44. Examples of Half-Life Isotope Half life C-15 2.4 sec Ra-224 3.6 days Ra-223 12 days I-125 60 days C-14 5700 years U-235 710 000 000 years

45. Learning Check NR3 The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 26 hours?

46. Solution NR3 t1/2 =13 hrs 26 hours = 2 x t1/2 Amount initial = 64mg Amount remaining = 64 mg x ½ x ½ = 16 mg

47. Ionizing Radiationradioactivity measurements High energy particles and photons that ionise atoms and molecules along their tracks in a medium are called ionizing radiation. For example, a, b, g, cosmic rays and X-rays are ionizing radiation. Most radioactive measurement are based on their ionizing effect. Ionizing radiation causes illness such as cancer and death. Radiation effect is a health and safety concern. Ionizing radiation can also be used in industry for various purposes. Light and microwaves that do not ionize atoms and molecules are called non-ionizing radiation.

48. Interaction of Heavy Charged Particles with Matter Fast moving protons, 4He, and other nuclei are heavy charged particles. Coulomb force dominates charge interaction. They ionize and excite(give energy to) molecules on their path.

49.  source Shield Scattering of Electrons in a Medium Fast moving electrons are light charged particles. They travel at higher speed., but scattered easily by electrons.

50. Factors that Determine Biological Effects of Radiation • The more energy the radiation has the larger its effect can be • The better the ionizing radiation penetrates human tissue, the deeper effect it can have • Gamma >> Beta > Alpha