1. Chapter 10: Radioactivity and Nuclear Processes Spencer L. Seager Michael R. Slabaugh www.cengage.com/chemistry/seager Jennifer P. Harris

2. Chapter 10 Objectives When you have completed your study of this chapter, you should be able to: Describe and characterize the common forms of radiation emitted during radioactive decay (Section 10.1; Exercise 10.2) Write balanced equations for nuclear reactions. (Section 10.2; Exercise 10.12) Define and understand the half ‐ life concept. (Section 10.3; Exercise 10.16) Describe the effects of radiation on health. (Section 10.4; Exercise 10.22) Describe, with examples, medical uses of radioisotopes. (Sections 10.6; Exercise 10.30) Describe, with examples, nonmedical uses of radioisotopes. (Sections 10.7; Exercise 10.36) Show that you understand the concept of induced nuclear reactions. (Section 10.8; Exercise 10.38) Describe the differences between nuclear fission and nuclear fusion reactions. (Section 10.9; Exercise (10.48)

3. RADIOACTIVE NUCLEI Radioactive nuclei are nuclei that undergo spontaneous changes and emit energy in the form of radiation. The emission of radiation by radioactive nuclei is often called radioactive decay. The intensity of radiation is unaffected by factors that normally influence the rates of chemical reactions.

4. TYPES OF RADIATION ALPHA RADIATION Alpha radiation consists of a stream of particles called alpha particles. Alpha particles are identical to helium-4 nuclei; they consist of two protons and two neutrons. BETA RADIATION Beta radiation consists of a stream of beta particles that are identical to electrons. They are created in the nucleus of radioactive atoms when a neutron is converted into a proton and an electron. GAMMA RADIATION Gamma radiation consists of high energy rays similar to X rays, but with a higher energy.

5. CHARACTERISTICS OF NUCLEAR RADIATION The characteristics of the common types of radiation along with the symbols used to represent them are summarized in the following table:

6. EQUATIONS FOR NUCLEAR REACTIONS In nuclear reactions, specific isotopes of an element may behave differently. For that reason, all particles involved in nuclear reactions are designated by the notation, where X is the symbol for the particle, A is the particle mass number, and Z is the particle atomic number or electrical charge. The notation is used for both specific isotopes and the different types of radiation. The equation for a nuclear reaction is balanced when the sum of the atomic numbers of the particles on the left side of the equation equals the sum of the atomic numbers of the particles on the right side, and the sum of the mass numbers on the left side equals the sum of the mass numbers on the right side.

7. EXAMPLES OF NUCLEAR REACTIONS Example 1: Bromine-84 decays by emitting a beta particle. What is the symbol for the daughter produced? Solution: The symbol for bromine-84 is. A beta particle has a mass number (the upper number) of 0, and a charge (the lower number) of -1. Thus, the daughter must have a mass number of 84 and an atomic number of 36. The element with an atomic number of 36 is krypton with a symbol of. The balanced equation is:

8. EXAMPLES OF NUCLEAR REACTIONS (continued) Example 2: When samarium-148 undergoes radioactive decay, the daughter produced is neodymium-144. What kind of radiation is emitted during the decay? Solution: The daughter has a mass number of 144, so the emitted radiation must have a mass number of 4. The difference between the atomic numbers is 2. Therefore, it is an alpha particle. The balanced equation is:

9. ISOTOPE HALF-LIFE The half-life of an isotope is the time required for one-half of the radioactive nuclei in a sample of the isotope to undergo radioactive decay. The half-life of an isotope is used to indicate stability. The longer the half-life, the more stable the isotope is.

10. EXAMPLES OF HALF-LIVES

11. THE HEALTH EFFECTS OF RADIATION The greatest danger to living organisms of exposure to long- term, low-level radiation is the ability of high-energy or ionizing radiation to dislodge electrons from molecules and generate highly reactive particles called radicals or free radicals. Free radicals are very reactive and may cause reactions to occur among stable materials in the cells of organisms such as genes and chromosomes. Such reactions might lead to genetic mutations, cancer, or other serious conditions. Short-term exposure to intense radiation results in tissue destruction in the exposed area and causes the symptoms of acute radiation syndrome.

12. PROTECTION AGAINST RADIATION EXPOSURE The use of shielding or distance are effective ways to prevent or minimize the exposure of individuals to harmful radiation. Shielding involves the placement of dense absorbing materials such as lead or concrete between the radiation source and individuals.

13. PROTECTION AGAINST RADIATION EXPOSURE (continued) Distance involves the use of the inverse square law of radiation which is a mathematical way of saying that the intensity of radiation is inversely proportional to the square of the distance from the source of the radiation. The mathematical equation is: According to this equation, a doubling of the distance from a radiation source will decrease the intensity of the radiation to ¼ the intensity at the original distance.

14. MEDICAL USES OF RADIOISOTOPES Radioactive isotopes can be detected easily in the body by using radiation detectors. Radioactive isotopes and nonradioactive isotopes of the same element undergo the same chemical reactions in the body. These two characteristics make radioactive isotopes useful as tracers in diagnostic medical work and as therapeutic agents in some medical treatments.

15. TRACERS/DIAGNOSTIC USE Tracers are radioisotopes used medically because their progress through the body or their localization in specific organs can be readily followed. Radioisotopes used as tracers should have as many of the following five characteristics as possible: Tracers should have short half-lives so they will decay while the diagnosis is being done but will give off as little radiation as possible after the diagnosis is completed. The daughter produced by the decaying tracer should be nontoxic and give off little or no radiation of its own. Ideally, it should be stable. The tracer should have a long enough half-life to allow it to be prepared and administered conveniently.

16. TRACERS If possible, the radiation given off by the tracer should be gamma rays because they penetrate tissue well and can be detected readily by detectors located outside the body. The tracer should have chemical properties that make it possible for the tissue being studied to either concentrate it in diseased areas and form a hot spot or essentially reject it from diseased areas and form a cold spot.

17. THERAPEUTIC USE OF RADIOISOTOPES Radioisotopes administered internally for therapeutic use should have as many of the following four characteristics as possible: The isotope should emit less penetrating alpha or beta radiation to restrict damage to the target tissue. The isotope half-life should be long enough to allow sufficient time for the desired therapy to be completed. The daughter of the isotope should be nontoxic and should give off little or no radiation. The target tissue should be able to concentrate the isotope to restrict the radiation damage to the target tissue.

18. EXAMPLES OF MEDICALLY USEFUL ISOTOPES

19. NONMEDICAL USES OF RADIOISOTOPES Many applications of radioisotopes have been made in diverse areas, including scientific studies, industry, and archeology. SCIENTIFIC STUDIES The study of photosynthesis in plants has been aided by the use of radioisotopes. During photosynthesis, plants combine carbon dioxide gas with water to form carbohydrates like starch and cellulose. Energy to drive the process is obtained from sunlight. The study of the chemical pathways followed by the carbon of CO 2 in photosynthesis has been greatly aided by using CO 2 that contains radioactive carbon-14.

20. NONMEDICAL USES OF RADIOISOTOPES (continued) ARCHEOLOGY The use of carbon dating to determine the age of artifacts is indispensable in some archeology studies. Radioactive carbon-14 forms naturally in the atmosphere. The radioactive carbon is converted to CO 2 gas which becomes incorporated into the cellulose of plants by photosynthesis. In living plants an equilibrium exists in which the plants contain the same fraction of radioactive carbon as the air. When the plant is cut down, carbon dioxide intake stops and the radioactive carbon in the plant begins to decay. The amount of carbon-14 in an artifact can be compared to the amount in air. The difference and the known half- life of carbon-14 (5600 years) allows the time the carbon- 14 has been decaying to be calculated, which is the age of the artifact.

21. NONMEDICAL USES OF RADIOISOTOPES (continued) Carbon-14 dating can only be used on objects less than about 50,000 years old. Other dating methods have developed. Potassium-40 undergoes electron capture to produce argon- 40. The half-life of potassium-40 is 1.3 x 10 9 years. By determining the amount of argon-40 in a potassium- containing mineral, it is possible to estimate the age of the mineral.

22. INDUCED NUCLEAR REACTIONS Induced nuclear reactions are reactions that take place when nuclei are bombarded with subatomic particles such as alpha particles or neutrons. An example of an induced nuclear reaction is the one that produces radioactive carbon-14 in the atmosphere. This process takes place when a nitrogen-14 atom is struck by a cosmic ray neutron. The reaction is: Induced nuclear reactions have been used to produce all of the transuranium elements in the periodic table (elements with atomic numbers greater than 92).

23. NUCLEAR ENERGY Nuclear energy is released in large amounts by the processes of nuclear fission or nuclear fusion. NUCLEAR FISSION Nuclear fission is a process in which large nuclei split into smaller, approximately equal-sized nuclei when hit by neutrons. During nuclear fission reactions, the total mass of the products of the reaction is less than the total mass of the reactants. The mass difference appears as energy in agreement with Einstein's famous equation E=mc 2, where the mass difference is m and c is the velocity of light.

24. NUCLEAR ENERGY (continued) A chain reaction is a nuclear reaction in which the products of one reaction cause a repeat of the reaction to take place. An expanding or branching chain reaction is a reaction in which the products of one reaction cause more than one more reaction to occur. A critical reaction is a constant rate reaction. A supercritical reaction is a branching chain reaction that will lead to an explosion.

25. NUCLEAR ENERGY (continued) A breeder reaction is a nuclear reaction in which isotopes that will not undergo spontaneous fission are changed into isotopes that will. ISSUE: can create more fuel than used up (can create a relatively inexpensive and non-polluting energy source) vs. fissionable product can be used for nuclear explosive devices (can make the world a less safe place)

26. NUCLEAR ENERGY (continued) NUCLEAR FUSION Nuclear fusion is a process in which small nuclei combine or fuse to form larger nuclei. As in nuclear fission reactions, the total mass of the reactants is greater than the total mass of the products, and the mass difference appears a energy in agreement with Einstein's equation. A thermonuclear reaction is a nuclear fusion reaction that requires a very high temperature to start. Research is being done to determine the possibility as an energy source. The overall reaction for the energy output of the sun is: