Sherril Soman Grand Valley State University

1. Lecture Presentation Chapter 19Radioactivity and Nuclear Chemistry Sherril Soman Grand Valley State University

2. Nuclear Medicine • Changes in the structure of the nucleus are used in many ways in medicine. • Nuclear radiation can be used to visualize or test structures in your body to see if they are operating properly. • For example, labeling atoms so their intake and output can be monitored • Nuclear radiation can also be used to treat diseases because the radiation is ionizing, allowing it to attack unhealthy tissue.

3. The Discovery of Radioactivity • Antoine-Henri Becquerel designed an experiment to determine if phosphorescent minerals also gave off X-rays. • Phosphorescence is the long-lived emission of light by atoms or molecules that sometimes occurs after they absorb light. • X-rays are detected by their ability to penetrate matter and expose a photographic plate.

4. Discovery of Radioactivity: Becquerel • Becquerel discovered that certain minerals were constantly producing energy rays that could penetrate matter. • Becquerel determined that 1. all the minerals that produced these rays contained uranium, and 2. the rays were produced even though the mineral was not exposed to outside energy. • He called them uranic raysbecause they were emitted from minerals that contained uranium. • Like X-rays • Not related to phosphorescence

5. Discovery of Radioactivity: Marie Curie • Marie Curie determined the rays were emitted from specific elements. • She also discovered new elements by detecting their rays. • Radium named for its green phosphorescence • Polonium named for her homeland • Because these rays were no longer just a property of uranium, she renamed it radioactivity.

6. Other Properties of Radioactivity • Radioactive rays can ionize matter. • Cause uncharged matter to become charged • Basis of Geiger counter and electroscope • Radioactive rays have high energy. • Radioactive rays can penetrate matter. • Radioactive rays cause phosphorescent chemicals to glow. • Basis of scintillation counter

7. What Is Radioactivity? • Radioactivity is the release of tiny, high-energy particles or gamma rays from an atom. • Particles are ejected from the nucleus.

8. Types of Radioactive Decay • Rutherford discovered three types of rays: • Alpha (a) rays • Have a charge of +2 and a mass of 4 amu • What we now know to be helium nucleus • Beta (b) rays • Have a charge of −1 c.u. and negligible mass • Electron-like • Gamma (g) rays • Form of light energy (not a particle like a and b) • In addition, some unstable nuclei emit positrons. • Like a positively charged electron • Some unstable nuclei will undergo electronscapture. • A low energy electron is pulled into the nucleus.

9. Rutherford’s Experiment g b a ++++++++++++ --------------

10. Penetrating Ability of Radioactive Rays a g b 0.01 mm 1 mm 100 mm Pieces of Lead

11. Facts about the Nucleus • Very small volume compared to volume of the atom • Essentially entire mass of atom • Very dense • Composed of protons and neutrons that are tightly held together • The particles that make up the nucleus are called nucleons.

12. Facts about the Nucleus • Every atom of an element has the same number of protons. • Atomic number (Z) • Atoms of the same elements can have different numbers of neutrons. • Isotopes • Different atomic masses • Isotopes are identified by their mass number (A). • Mass number = number of protons + neutrons

13. Facts about the Nucleus • The number of neutrons is calculated by subtracting the atomic number from the mass number. • The nucleus of an isotope is called a nuclide. • Less than 10% of the known nuclides are nonradioactive; most are radionuclides. • Each nuclide is identified by a symbol. • Element – mass number = X – A

14. Radioactivity • Radioactive nuclei spontaneously decompose into smaller nuclei—radioactive decay. • We say that radioactive nuclei are unstable. • Decomposing involves the nuclide emitting a particle and/or energy. • The parent nuclideis the nucleus that is undergoing radioactive decay. • The daughter nuclide is the new nucleus that is made. • All nuclides with 84 or more protons are radioactive.

15. Important Atomic Symbols

16. Transmutation • Rutherford discovered that during the radioactive process, atoms of one element are changed into atoms of a different element—transmutation. • Showing that statement three of Dalton’s atomic theory is not valid all the time, only for chemical reactions • For one element to change into another, the number of protons in the nucleus must change!

17. Chemical Processes versus Nuclear Processes • Chemical reactions involve changes in the electronic structure of the atom. • Atoms gain, lose, or share electrons. • No change in the nuclei occurs. • Nuclear reactions involve changes in the structure of the nucleus. • When the number of protons in the nucleus changes, the atom becomes a different element.

18. Nuclear Equations • We describe nuclear processes with nuclear equations. • Atomic numbers and mass numbers are conserved. • The sum of the atomic numbers on both sides must be equal. • The sum of the mass numbers on both sides must be equal.

19. Alpha Decay • Occurs when an unstable nucleus emits a particle composed of two protons and two neutrons • Most ionizing, but least penetrating of the types of radioactivity • Loss of an alpha particle means • the atomic number decreases by 2, and • the mass number decreases by 4.

21. Beta Decay • Occurs when an unstable nucleus emits an electron • About 10 times more penetrating than α, but only about half the ionizing ability • When an atom loses a β particle its • atomic number increases by 1, and • the mass number remains the same.

23. Gamma Emission • Gamma (g) rays are high energy photons of light. • No loss of particles from the nucleus • No change in the composition of the nucleus • Same atomic number and mass number • Least ionizing, but most penetrating • Generally occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange

24. Positron Emission • Positron has a charge of +1 and a negligible mass. • Antielectron • Similar to beta particles in their ionizing and penetrating ability • When an atom loses a positron from the nucleus, its • mass number remains the same, and • its atomic number decreases by 1. • Positrons result from a proton changing into a neutron.

26. Electron Capture • Occurs when an inner orbital electron is pulled into the nucleus. • No particle emission, but atom changes • Same result as positron emission • When a proton combines with the electron to make a neutron its • mass number stays the same, and • its atomic number decreases by 1.

28. Nuclear Equations • In the nuclear equation, mass numbers and atomic numbers are conserved. • We can use this fact to determine the identity of a daughter nuclide if we know the parent and mode of decay.

29. What Causes Nuclei to Decompose? • The particles in the nucleus are held together by a very strong attractive force only found in the nucleus called the strong 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 don’t repel each other like the protons do.

30. 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, neutrons are converted to protons via b decay. • If the N/Z ratio is too low, protons are converted to neutrons via positron emission or electron capture. • Or via a decay, though not as efficiently

31. Valley of Stability For Z = 1 ⇒ 20, stable N/Z ≈ 1.

32. Valley of Stability For Z = 1 ⇒ 20, stable N/Z ≈ 1. For Z = 20 ⇒40, stable N/Zapproaches 1.25.

33. Valley of Stability For Z = 1 ⇒ 20, stable N/Z ≈ 1. For Z = 20 ⇒ 40, stable N/Zapproaches 1.25. For Z = 40 ⇒ 80, stable N/Zapproaches 1.5.

34. Valley of Stability For Z = 1 ⇒ 20, stable N/Z ≈ 1. For Z = 20 ⇒ 40, stable N/Zapproaches 1.25. For Z = 40 ⇒ 80, stable N/Zapproaches 1.5. For Z > 83, there are no stable nuclei.

35. Magic Numbers • Besides the N/Z ratio, the actual numbers of protons and neutrons affect stability. • Most stable nuclei have even numbers of protons and neutrons. • 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. • Same principle as stability of the noble gas electron configuration • Most stable when N or Z = 2, 8, 20, 28, 50, 82; or N = 126

36. Decay Series • In nature, often one radioactive nuclide changes into another radioactive nuclide. • That is, the daughter nuclide is also radioactive. • All atoms with Z > 83 are radioactive. • All of the radioactive nuclides that are produced one after the other until a stable nuclide is made is called a decay series.

37. U-238 Decay Series

38. Natural Radioactivity • There are small amounts of radioactive minerals in the air, ground, and water. • They are even in the food you eat! • The radiation you are exposed to from natural sources is called background radiation.

39. Detecting Radioactivity Particles emitted by radioactive nuclei have a lot of energy and therefore can be readily detected. • Radioactive rays can expose light-protected photographic film. • We may use photographic film to detect the presence of radioactive rays—film badge dosimeters.

40. Detecting Radioactivity • Radioactive rays cause air to become ionized. • An electroscope detects radiation by its ability to penetrate the flask and ionize the air inside. • AGeiger-Müller counterworks by counting electrons generated when Ar gas atoms are ionized by radioactive rays.

41. Detecting Radioactivity • Radioactive rays cause certain chemicals to give off a flash of light when they strike the chemical. • A scintillation counteris able to count the number of flashes per minute.

42. Rate of Radioactive Decay • The rate of change in the amount of radioactivity is constant, and is different for each radioactive “isotope.” • Change in radioactivity measured with Geiger counter • Counts per minute • Each radionuclide had a particular length of time it required to lose half its radioactivity—a constant half-life. • We know that processes with a constant half-life follow first order kinetic rate laws. • The rate of radioactive change was not affected by temperature. • In other words, radioactivity is not a chemical reaction!

43. Kinetics of Radioactive Decay • Rate = kN • N = number of radioactive nuclei • The shorter the half-life, the more nuclei decay every second; therefore, we say the sample is “hotter.”

44. Half-Lives of Various Nuclides

45. Half-Life Half of the radioactive atoms decay each half-life.

46. Kinetics of Radioactive Decay • Radioactive decay is a first order process. • Nt = number of radioactive nuclei at time, t • N0 = initial number of radioactive nuclei

47. Radiometric Dating • The change in the amount of radioactivity of a particular radionuclide is predictable and not affected by environmental factors. • By measuring and comparing the amount of a parent radioactive isotope and its stable daughter, we can determine the age of the object. • Using the half-life and the previous equations • Mineral (geological) dating • Compare the amount of U-238 to Pb-206 in volcanic rocks and meteorites. • Dates Earth to between 4.0 and 4.5 billion years old • Compare amount of K-40 to Ar-40.

48. Radiocarbon Dating • All things that are alive or were once alive contain carbon. • Three isotopes of carbon exist in nature, one of which, C-14, is radioactive. • C-14 radioactive with half-life = 5730 years • Atmospheric chemistry keeps producing C-14 at nearly the same rate it decays.

49. Radiocarbon Dating • While still living, C-14/C-12 is constant because the organism replenishes its supply of carbon. • CO2 in air is the ultimate source of all C in an organism. • Once the organism dies the C-14/C-12 ratio decreases. • By measuring the C-14/C-12 ratio in a once living artifact and comparing it to the C-14/C-12 ratio in a living organism, we can tell how long ago the organism was alive. • The limit for this technique is 50,000 years old. • About 9 half-lives, after which radioactivity from C-14 will be below the background radiation

50. Nonradioactive Nuclear Changes • Fission • – The large nucleus splits into two smaller nuclei. • Fusion • –Small nuclei can be accelerated to smash together to make a larger nucleus. • Both fission and fusion release enormous amounts of energy. • Fusion releases more energy per gram than fission.