Chapter 19 Radioactivity and Nuclear Chemistry

1. Chapter 19Radioactivity and Nuclear Chemistry

2. GOALS Types of radioactivity Identify radioactive nuclides Nuclear equations Binding energy; per nucleon; units Kinetics of radioactive decay

3. Facts About the Atomic Nucleus • Every atom of an element has the same number of protons (+ve) • atomic number (Z) • Atoms of the same elements can have different numbers of neutrons (no charge) • Isotopes: atoms of the same element, having same atomic number, Z, but different mass number, A (diff no. of neutrons). • Isotopes are identified by their mass number (A) • mass number = number of protons + neutrons

4. Facts About the Atomic Nucleus • mass number = number of protons + neutrons neutrons = mass number – number of protons • The nucleus of an isotope is called anuclide • Each nuclide is identified by a symbol

5. 11B 10B Nuclide Symbols • Boron-10 (105B) has 5 p and 5 n • Boron-11 (115B) has 5 p and 6 n • Oxygen-16 (168O) has 8 p and 8 n • Oxygen-17 (178O) has 8 p and 9 n • Oxygen-18 (188O) has 8 p and 10 n

6. The Discovery of Radioactivity • Becquerel discovered that certain minerals were constantly producing penetrating energy rays he called uranic rays (1896) Marie Curie discovered 2 new elements (Po, Ra) which also emitted uranic rays. • Curie changed term uranic rays to radioactivity (present in elements other than uranium). Some nuclei are unstable; they emit particles and/or electromagnetic radiation spontaneously. This is radioactivity.

7. Types of Radioactive Rays • Rutherford discovered there were 3 types of radioactivity; • 2 additional types were later discovered. • alpha (a) & beta (b) decay, gamma ray (g); then positron emission, and electron capture. Another type of radioactivity (nuclear transmutation) results from the bombardment of nuclei (heavy) by neutrons, protons or other nuclei (lighter).

8. a g b Penetrating Ability of Radioactive Rays 0.01 mm 1 mm 100 mm Pieces of lead

9. Important Atomic Symbols

10. Nuclear Equations • nuclear processes are described using nuclear equations • use the symbol of the nuclide to represent the nucleus • atomic numbers and mass numbers are conserved • use this to predict identity of daughter nuclide if parent and emitted particle are known emitted particle: product captured particle: reactant

11. Alpha Emission • an  particle contains 2 protons and 2 neutrons • most ionizing, but least penetrating • loss of an alpha particle means • atomic number decreases by 2 • mass number decreases by 4

12. Beta Emission • An unstable nucleus emits an electron • when an atom loses a  particle its • atomic number increases by 1 • mass number remains the same • in beta decay, a neutron changes into a proton

13. If californium-251 decays by successive α, α, β emissions, what nucleus is produced? a)b) c) d) e)

14. Gamma Emission • gamma (g) rays are high energy photons of light • least ionizing, but most penetrating • generally occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange

15. Positron Emission • The positron has a charge of +1 and negligible mass • anti-electron • when an atom loses a positron from the nucleus, its • mass number remains the same • atomic number decreases by 1 • A positron appears to result from a proton changing into a neutron

16. Electron Capture • occurs when an inner orbital electron is pulled into the nucleus • no particle emission, but atom changes • same result as positron emission • proton combines with the electron to make a neutron • mass number stays the same • atomic number decreases by one

17. Summary of Decay Processes (Table 19.1; pg 871)

18. Write the nuclear equation for positron emission from K-40 a) Write the nuclide symbols for both the starting radionuclide and the particle b) Set up the equation (emitted particles are products; captured particles are reactants) c) Determine the mass number and atomic number of the missing nuclide (mass and atomic numbers are conserved)

19. Write the nuclear equation for positron emission from K-40 4) Determine the element from the atomic number Q. In a decay series, U-238 emits 8 alpha particles and 6 beta particles. What nuclide is formed? Mass dec by 32; charge = +6 & -16

20. Write a nuclear equation for each of the following alpha emission from U-238 beta emission from Ne-24 positron emission from N-13 electron capture by Be-7

21. Stability of Nuclei - stable isotopes fall in a very narrow range called the island of stability.

22. What Causes Nuclei to Break Down? • the particles in the nucleus are held together by a very strong attractive force found in the nucleus called thestrong force • acts only over very short distances • the neutrons play an important role in stabilizing the nucleus, as they add to the strong force, but do not repel each other like the protons do

23. Neutron to Proton (N/Z) Ratio • the ratio of neutrons : protons is an important measure of the stability of the nucleus • if the N/Z ratio is too high (neutron rich) – neutrons are converted to protons via b decay • if the N/Z ratio is too low (proton rich) – protons are converted to neutrons via positron emission or electron capture • or via a decay – though not as efficient

24. Valley (Island) of Stability (Plot of # Neutrons vs # Protons) for Z = 1  20 (H - Ca), stable N/Z ≈ 1 for Z = 20  40, stable N/Z approaches 1.25 for Z = 40  80, stable N/Z approaches 1.5 Heavy nuclei: for Z > 83, there are no stable nuclei low N/Z

25. Determine the kind of radioactive decay that Mg-22 undergoes • Mg-22 • Z = 12 (protons) • N = 22 – 12 = 10 (neutrons) • N/Z = 10/12 = 0.83 • from Z = 1  20, stable nuclei have N/Z ≈ 1 • Mg-22 has low N/Z; it should convert 11p into 10n, therefore it will undergo positron emission or electron capture

26. Determine the kind of radioactive decay that N-18 undergoes • N-18 • Z = 7 (protons) • N = 18 – 7 = 11 (neutrons) • N/Z = 11/7 = 1.57 • from Z = 1  20, stable nuclei have N/Z ≈ 1

27. Q. Which of the following will undergo betadecay? 16O, 20F, 13N

28. besides the N/Z ratio, the numbers of protons and neutrons effects stability Magic Numbers most stable nuclei have even numbers of protons and neutrons most stable when N or Z = 2 (He), 8 (O), 20 (Ca), 28 (Ni), 50 (Sn), 82 (Pb) only a few have odd numbers of protons and neutrons if the total number of nucleons adds to a magic number, the nucleus is more stable (compare # electrons in noble gases)

29. Binding Energy, Eb -All atoms are a little lighter than they are really supposed to be. Missing mass: ∆m = mass defect. -This missing mass is converted to energy, and released when 1 mole of atoms is formed from its subatomic particles (protons + neutrons + electrons). -Energy holds the nucleus together.

30. Calculating Binding Energy, Eb Eb is the energy required to separate the nucleus of an atom into protons, neutrons, electrons. For stability, Eb > electrostatic repulsive forces between protons. In deuterium, 21H 21H 11p + 10n Eb = 2.15  108 kJ/mol 21H Eb per mol nucleon = Eb/2 nucleons = 1.08  108 kJ/mol nucleons Also, calc Eb per nucleon (6.022  1023 nucleons)

31. Calculating Binding Energy, Eb For deuterium, 21H: 21H 11p + 10n Actual mass of 21H = 2.01410 g/mol (given or PT) Mass of proton = 1.007825 g/mol Mass of neutron = 1.008665 g/mol Theoretical mass = 2.016490 g/mol Mass defect (‘missing mass’) = 2.016490 – 2.01410 = 0.00239 g/mol

32. Calculate Binding Energy, Eb Mass defect = 0.00239 g/mol = (0.002391000) kg/mol = 2.39  10-6 kg/mol From Einstein’s equation: Eb = (∆m)c2 = 2.39  10-6 kg  (3.00 × 108 m/s)2 = 2.15 ×1011 kgm2/s2(but 1 kgm2/s2 = 1 J) = 2.15  1011 J/mol 1000 J = 2.15  108 kJ/mol Two nucleons for deuterium, 21H: 11p + 10n Eb /mol nucleon = 1.08  108 kJ/mol nucleons

33. Calculating Binding Energy, Eb For I-127, 12753I: 53p + 74n (i.e. 127 nucleons) Actual mass of 12753I = 126.9045 g/mol (given or PT) 53 protons = 531.007825 g/mol = 53.41473 g/mol 74 neutrons = 74  1.008665 g/mol = 74.64121 g/mol Theoretical mass defect = 128.05594 g/mol Mass defect = (128.05594 -126.9045) g/mol = 1.1514 g/mol = 1.1514  10-3 kg/mol

34. Calculate Binding Energy, Eb Eb = 1.1514  10-3 kg/mol (3.00 × 108 m/s)2 = 1.04 ×1014 kgm2/s2(but 1 kgm2/s2 = 1 J) = 1.04 ×1014 J/mol Eb /mol nucleon = 1.04 ×1014 J/ (127 nucleons) = 8.19 ×1011 J Eb /nucleon = 8.19×1011 J  (6.022 ×1023) = 1.36 ×10-12 J Also, can express Eb inMeV: 1 MeV = 1.602 ×10-13 J Eb /nucleon = ? MeV

35. Plot of Ebvs Mass • -the greater the binding energy per nucleon, the more stable the nucleus is

36. Nuclear Fission The splitting of a heavy unstable nucleus of an atom into two or more fragments; Pu, U & Th! -induced reaction to produce energy! Energy released  16,800,000,000 kJ/mol (235 g Uranium)

37. Nuclear Fusion Light nuclei fuse to generate heavier nuclei (more stable) Free of long-lived radioactive waste. More difficult to achieve. Nuclei must travel at v. large KE’s at each other. More destructive than fission bombs (WWII)!

38. Kinetics of Radioactive Decay • Rate = kN It is a first order process • N = number of radioactive nuclei the shorter the half-life, the more nuclei decay every second (sample is hot!), the higher the rate

39. t1/2 k + m0, t mt The half life of Pu-236 is 2.86 years. Starting with a 1.35 mg sample of Pu-236, calculate the mass that will remain after 5.00 years Given: Find: mass Pu-236 = 1.35 mg, t = 5.00 yr, t1/2 = 2.86 yr mass, mg Concept Plan: Relationships:

40. t1/2 k + m0, t mt Starting with a 1.35 mg sample of Pu-236, calculate the mass that will remain after 5.00 years Solve: Check: units are correct, the magnitude makes sense since it is less than ½ the original mass for longer than 1 half-life

41. t1/2 k + rate0, ratet t An ancient skull gives 4.50 dis/min∙gC. If a living organism gives 15.3 dis/min∙gC, how old is the skull? 14C-t1/2 = 5730 yr dis = disintegrations Given: Find: ratet = 4.50 dis/min∙gC, ratet = 15.3 dis/min∙gC time, yr Concept Plan: Relationships: Solve:

42. An ancient skull gives 4.50 dis/min∙gC. If a living organism gives 15.3 dis/min∙gC, how old is the skull? 14C-t1/2 = 5730 yr Solve: Check: units are correct, the magnitude makes sense since it is less than 2 half-lives

43. An artifact containing carbon taken from the tomb of a king of ancient Egypt gave 8.1 dpm/gC. How old is the artifact? Carbon from a living organism gives 15.3 dis/min∙gC; 14C-t1/2 = 5730 yr.dis = disintegrations

44. Joliot-Curies Artificial Nuclear Reactions • bombardment of one nucleus with another (2H,4He, 10B, 12C)causing new atoms to be made • can also bombard with neutrons; protons • reaction done in a particle accelerator • linear • cyclotron Tc-97 is made by bombarding Mo-96 with deuterium, releasing a neutron

45. Artificial Nuclear Reactions Reactions using neutrons are called n,g reactions because a g ray is usually emitted. Radioisotopes used in medicine are often made by n,g reactions. An example of a n,g reactionis production of radioactive 31P for use in studies of P uptake in the body. 3115P + 10n 3215P + g

46. Transuranium Elements Elements beyond 92 (transuranium) made starting with an n, g reaction 23892U + 10n 23992U + g 23992U 23993Np + 0-1b 23993Np 23994Pu + 0-1b

47. Q. 56Fe when bombarded with deuterium, produces 54Mn and one other particle. Write a balanced equation for the reaction & identify the other particle. • 5626He + 21H  5425Mn + ?

48. Medical Uses of Radioisotopes

49. Nonmedical Uses of Radioactive Isotopes • smoke detectors • Am-241 • smoke blocks ionized air, breaks circuit • insect control • sterilize males • food preservation • radioactive tracers • follow progress of a “tagged” atom in a reaction

50. Nonmedical Uses of Radioactive Isotopes • authenticating art object • many older pigments and ceramics were made from minerals with small amounts of radioisotopes • crime scene investigation • measure thickness or condition of industrial materials • corrosion • track flow through process • gauges in high temp processes • weld defects in pipelines • road thickness