1. In chemical reactions, only the outer electrons of the atoms are disturbed. In nuclear reactions, the nuclear changes that occur are independent of the chemical environment of the atom.

2. You have two samples of water, each made up of different isotopes of hydrogen: one contains hydrogen-1 and the other contains hydrogen-3. a.Would you expect these two water samples to be chemically similar? b.Would you expect these two water samples to be physically the same? c.Which one of these water samples would you expect to be radioactive?

3. a.Yes, isotopes have similar chemical properties. b.No, the hydrogen-3 water has more mass than the hydrogen-1 water. c.The hydrogen-3 (tritium) water should be radioactive.

4. Radioactive decay is the process in which a nucleus spontaneously disintegrates, giving off radiation. A nuclear bombardment reaction is a nuclear reaction in which a nucleus is bombarded, or struck, by another nucleus or by a nuclear particle.

5. The phenomenon of radioactivity was discovered by Henri Becquerel in 1896. Becquerel noted that photographic plates had bright spots when they were exposed to uranium minerals. This radiation was found to be composed of three types when exposed to an electric field.

6. We write nuclear equations using nuclide symbols. Nuclear equations are balanced when the total mass number and the atomic number on both the reactant and product sides are equal. Let’s look at the decay of uranium-238.

7. Symbols for other particles are given below:

8. An alpha particle is a helium nucleus. Alpha emission occurs when a nucleus decays by emitting an alpha particle, 2 protons and 2 neutrons (can be blocked by a sheet of paper). A beta particle is an electron. Beta emission occurs when a nucleus decays by emitting a beta particle, an electron (can be stopped by a sheet of aluminum). A positron is similar to an electron, but has a positive charge. Positron emission occurs when a nucleus decays by emitting a positron. A gamma photon is a particle of electromagnetic radiation that has higher energy and a smaller wavelength than an x- ray (can be blocked by several centimeters of lead).

9. There are six common types of radioactive decay. 1.Alpha emission (too many p + and n o ) Emission of an alpha particle from an unstable nucleus.

10. 2.Beta emission (too many neutrons) Emission of a beta particle from an unstable nucleus. Beta emission is equivalent to a neutron converting to a proton.

11. 3.Positron emission (if there are too many p + ) Emission of a positron particle from an unstable nucleus. Positron emission is equivalent to a proton converting to a neutron.

12. 4.Electron capture (if there are too many p + ) The decay of an unstable nucleus by capture of an electron from an inner orbital of the atom. Electron capture is equivalent to a proton converting to a neutron.

13. 5.Gamma emission Emission from an excited nucleus of a gamma photon, corresponding to radiation with a wavelength of approximately 10 -12 m. Technetium-99m is an example of a metastable nucleus; it is in an excited state and has a lifetime of ≥ 10 -9 s.

14. 6.Spontaneous fission The spontaneous decay of an unstable nucleus in which a heavy nucleus of mass number greater than 89 splits into lighter nuclei and energy is released.

15. ? Radon-222 is a radioactive noble gas that is sometimes present as an air pollutant in homes built over soil with high uranium content (uranium-238 decays to radium-226, which in turn decays to radon-222). A radon-222 nucleus decays to polonium-218 by emitting an alpha particle. Write the nuclear equation for this decay process.

16. The atomic number of radon is 86. The atomic number of polonium is 84.

17. ? Iodine-131 is used in the diagnosis and treatment of thyroid cancer. This isotope decays by beta emission. What is the product nucleus?

18. The atomic number of iodine is 53.

19. Nuclear Stability It is reasonable to wonder how a nucleus with positively charged protons is held together, given that positively charged particles repel each other. The stability of the nucleus is due to the strong nuclear force. The nuclear force acts only at very short distances, about 10 -13 m. At this distance it is stronger than the electric repulsion.

20. A plot of number of protons versus number of neutrons for each stable nuclide yields a band of stability, the region in which stable nuclides lie.

21. For stable nuclides with Z ≤ 20, the ratio of neutrons to protons is between 1 and 1.1.

22. For stable nuclides with Z > 20, the ratio of neutrons to protons increases to about 1.5. This is believed to be due to the increasing repulsion between protons, which requires more neutrons to increase the strong nuclear force.

23. 0β0β Beta emission reduces the N/Z ratio by converting a neutron to a proton.

24. 0β0β 1 Either process increases the N/Z ratio by converting a proton to a neutron.

25. 4α4α 2

27. ? Thallium-201 is a radioactive isotope used in the diagnosis of circulatory impairment and heart disease. How do you expect it to decay? Thallium-201 has 81 protons and 120 neutrons. N/Z < 1.5 (too small). Thallium-201 will decay by either electron capture or positron emission—probably electron capture, given that it is a heavy element.

28. Radioactive Decay Series Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. A sequence in which one radioactive nucleus (parent atom) decays to a second (daughter isotope), which then decays to a third, and so forth, until a stable nucleus of lead is formed. Three radioactive decay series are found naturally: uranium-238, uranium-235, and thorium-232.

29. Copyright © Cengage Learning. All rights reserved.20 | 29 The radioactive decay series for uranium-238 ends with lead-206

30. Nuclear Bombardment Reactions Nuclear bombardment reactions are not spontaneous. They involve the collision of a nucleus with another particle. Transmutation is the change of one element into another by bombarding the nucleus of the element with nuclear particles or nuclei.

31. When Rutherford allowed alpha particles to collide with nitrogen nuclei, he found that a proton was ejected and oxygen was formed.

32. James Chadwick proposed the existence of the neutron based on the result of bombarding beryllium-9 with alpha particles. The product included neutral radiation we now know as neutrons. The first radioactive nucleus produced in the laboratory was phosphorus-30. Phosphorus-30 decays by positron emission.

33. When heavy nuclei are bombarded, the bombarding particles are scattered or deflected. To produce transmutation, the bombarding particles must be accelerated. A particle accelerator is a device used to accelerate electrons, protons, alpha particles, and other ions to very high speeds.

34. A cyclotron is a type of particle accelerator consisting of two hollow, semicircular metal electrodes called dees (because the shape resembles the letter D), in which charged particles are accelerated by stages to higher and higher kinetic energies.

36. Transuranium elements are elements with atomic numbers greater than that of uranium (Z = 92), the naturally occurring element of greatest atomic number. In 1940, the first transuranium element was produced at the University of California, Berkeley, when element 93 (later named neptunium) was documented. It was created by bombarding uranium-238 with neutrons, producing uranium- 239, which then decayed by beta emission to give neptunium-239.

37. Radiations and Matter Radiation from nuclear processes affects matter in part by dissipating energy in it. The dissipation can ionize atoms and molecules and, in some cases, excite electrons in matter. When these electrons undergo transitions to their ground states, light is emitted. Because nuclear radiations can form ions and break chemical bonds, they adversely affect biological organisms.

38. Radiation Counters There are two types of devices: ionization counters and scintillation counters.

39. The Geiger counter is an ionization counter used to count particles emitted by radioactive nuclei. It consists of a metal tube filled with gas, such as argon.

40. A scintillation counter detects nuclear radiation based on flashes of light generated in a material by the radiation. A phosphor is a substance that emits flashes of light when struck by radiation. In the scintillation counter, the flashes of light are detected by a photomultiplier tube.

41. The activity of a radioactive source is the number of nuclear disintegrations per unit time occurring in a radioactive material. The curie (Ci) is a unit of activity equal to 3.700 × 10 10 disintegrations per second.

42. Biological Effects and Radiation Dosage The rad (from radiation affected dose) is the dosage of radiation that deposits 1 × 10 -2 J of energy per kilogram of tissue.

43. The rem is a unit of radiation dosage that is used to relate various kinds of radiation in terms of biological destruction. It equals the rad times a factor for the type of radiation, called the relative biological effectiveness (RBE). rem = rad × RBE Beta and gamma radiation have an RBE of about 1, neutron radiation has an RBE of about 5, and alpha radiation has an RBE of about 10.

44. The effect of radiation on a person depends on the dosage and the length of time of the exposure. A series of small doses have less overall effect than a large dose given all at once. A single dose of 500 rems is fatal to most people. Detectable effects are seen at dosages as low as 30 rems. Background radiation averages about 0.1 rem per year but varies dramatically by location.

48. ? If you are internally exposed to 10 rads of , , and  radiation, which form of radiation will cause the greatest damage? The  radiation has the highest RBE, so it will cause the greatest damage.

49. Half-life is the time it takes for one-half of the nuclei in a sample to decay. In each half-life problem there are basically four variables: total time half-life starting amount ending amount Half-lives can range from microseconds to thousands of years and is characteristic of each substance. n = number of half-lives

51. After one half-life, half of the sample (0.5) remains. After two half-lives, one-fourth of the sample (0.25) remains. After three half-lives, one-eighth of the sample remains. This relationship is summarized in the following equation and in the graph on the next slide.

53. ? Thallium-201 is used in the diagnosis of heart disease. This isotope decays by electron capture; the decay constant is 2.63 × 10 -6 /s. What is the half-life of thallium-201 in days?

54. t 1/2 = 0.693s1 min1 hr1 day 2.63 x 10 -6 60 s60 min24 hr

55. Radioactive Dating Because the rate of radioactive decay is constant, this rate can serve as a sort of clock for dating objects. Carbon-14 is part of all living material. While a plant or animal is living, the fraction of carbon-14 in it remains constant due to exchange with the atmosphere. Once dead, the fraction of carbon-14 and, therefore, the rate of decay decrease. In this way, the fraction of carbon-14 present in the remains becomes a clock measuring the time since the plant’s or animal’s death.

56. The half-life of carbon-14 is 5730 years. Living organisms have a carbon-14 decay rate of 15.3 disintegrations per minute per gram of total carbon. The ratio of disintegrations at time t to time 0 is equal to the ratio of nuclei at time t to time 0.

58. ? Why do you think that carbon-14 dating is limited to materials that are less than 50,000 years old? After 50,000 years, about ten half-lives would have passed, meaning there would be almost no carbon-14 present to detect and measure. (Only about 0.1% carbon-14 would remain.)

59. Applications of Radioisotopes: Medical Therapy and Diagnosis Radioisotopes are used for diagnosis of many medical conditions. For example, they are used to develop images of internal body organs so those organs’ functioning can be examined. More than 100 different radioactive isotopes have been used in medicine. Radioimmunoassay is a technique for analyzing blood and other body fluids for the presence of very small quantities of biologically active substances.

61. Nuclear fission is a nuclear reaction in which a heavy nucleus splits into lighter nuclei and releases energy. This process sometimes occurs spontaneously, as with californium-252.

62. In other cases, a nucleus undergoes fission after being bombarded by neutrons. (aka fissionable material) When bombarded by a neutron, uranium-238 gives three possible sets of products.

64. Nuclear Fission; Nuclear Reactors When the uranium-235 nucleus splits, it releases two or three neutrons. These neutrons are absorbed by other uranium-235 nuclei, which then release even more neutrons. A nuclear chain reaction is a self-sustaining series of nuclear fissions caused by the absorption of neutrons released from previous nuclear fissions. If a chain reaction is not controlled, a nuclear explosion can occur.

65. Representation of a chain reaction of nuclear fissions.

66. To sustain a chain reaction in a sample of fissionable material, a minimum amount of the particular fissionable material is needed—the critical mass. If the mass is much larger (a supercritical mass), the number of nuclei that split will multiply rapidly. An atomic bomb is detonated by creating a supercritical mass of fissionable material.

67. A nuclear fission reactor is a device that permits a controlled chain reaction of nuclear fission. Fuel rods contain the fissionable material. They alternate with control rods that absorb neutrons.

69. Nuclear Fusion Energy is released when light nuclei combine into a heavier nucleus in a fusion reaction. These reactions have been observed in the laboratory using particle accelerators. For the nuclei to react, the bombarding nuclei must have enough kinetic energy to overcome the repulsion between positive nuclei. The energy required is not practically available at this time.

70. Nuclear fusion is a nuclear reaction in which light nuclei combine to give a more stable, heavier nucleus plus possibly several neutrons. This process releases energy.

71. 2 3