Radioactivity and Nuclear Reactions

1. Radioactivity and Nuclear Reactions Chapter 18 Physical Science

2. Section 1: Radioactivity • Why is it important? • Radioactivity is everywhere because every element on the periodic table has some atomic nuclei that are radioactive. • New Vocabulary that you will learn in this section: • Strong force • radioactivity

3. What you’ll learn… • Describe the structure of an atom and its nucleus • Explain what radioactivity is. • Contrast properties of radioactive and stable nuclei • Discussthe discovery of radioactivity.

4. The nucleus…lets review • Atoms are composed of protons, neutrons, and electrons. • Nucleus occupies only a tiny fraction of the space in the atom. Contains almost all the mass of the atom • contains protons (+) and neutrons (neutral) • Total amount of protons= atomic number (charge in a nucleus) • Electrons-located outside the nucleus (-) charge

5. The Strong Force (p537) • How do you suppose protons and neutrons are held together so tightly in the nucleus?

6. Answer • The strong force causes protons and neutrons to be attracted to each other • See Figure 2 on page 537

7. The Strong Force cont… • 1 of the 4 basic forces in nature and is about 100 times stronger than the electric force • Electric force: long range force, so protons that are far apart still are repelled by the electric force • The total force between two protons/neutrons depends on how far apart they are. • The strong force is a short-range force: that quickly becomes extremely weak as protons and neutrons get farther apart

8. Attraction and Repulsion • If a nucleus has only a few protons and neutrons, they are all close enough together to be attracted to each other by the strong force. • See Figure 4A • Protons and neutrons are held together less tightly in a large nuclei. (each protons and neutron is attracted to only a few neighbors by the strong force) • See Figure 4B • All protons is a large nucleus exert a repulsive electric force on each other. Thus, the electric repulsive force on a proton in a large nucleus is larger than it would be in a small nucleus

9. Radioactivity p. 538 • When the strong force is not large enough to hold a nucleus together tightly, the nucleus can decay and give off matter and energy. This process of nuclear decay is called Radioactivity. • Large nuclei tend to be unstable and can break apart of decay. • All nuclei that contain more than 83 protons are radioactive. See 2nd paragraph (read as a class) • Almost all elements with more than 92 protons do not exist naturally on Earth. Produced in labs (synthetic elements)

10. Isotopes (p. 539) • Atoms of the same element that have different numbers of neutrons but the same number of protons • Ex). The elements Carbon • 3 isotopes that occur naturally (Carbon nuclei can have 6,7, or 8 neutrons) • Look at figure 5 and identify the ratio of protons to neutrons in each isotope of helium • Answer: Helium-3 : 2 to 1; Helium-4 : 2 to 2 Brain Pop Video: http://glencoe.mcgraw-hill.com/sites/0078779626/student_view0/brainpop_movies.html#

11. Stable and Unstable Nuclei • The ratio of neutrons to protons is related to the stability of the nucleus • Less massive elements are stable when: • Ratio is 1 : 1 • Heavier elements are stable when: • Ratio is 3 : 2 • Nuclei is any isotopes that differ much from these ratios are unstable. (whether the elements are light or heavy) • Nuclei with too many or too few neutrons compared to the number of protons are radioactive

12. Nucleus Numbers • Atomic #: # of protons in nucleus • Mass #: # of protons and neutrons • See page 539 at the bottom

13. Mini Lab • 15 silver=NEUTRONS • 13 green= protons • 2 red= 2 nuclei • Small Nucleus Model: • Place 2 green protons and 3 silver neutrons around a red nucleus so they touch • Large Nucleus Model: • Arrange the remaining candies around the other red nucleus so they are touching.

14. Analysis/Conclusion Questions: • Compare the number of protons and neutrons touching a green protons in both models. • (How many red and yellow are touching a green) • Suppose the strong force on a green proton is due to protons and neutrons that touch it. Compare the strong force on a green proton in both models.

15. Answers: • 1. About the same number touch the green protons in the larger nucleus, so the strong force is about the same. • 2. The total number of protons and neutrons increases. The number nearby stays the same. The electromagnetic force on a proton increases, but the strong force stays the same, so the nucleus is less stable.

16. The Discovery of Radioactivity (p. 540) • 1896 Henri Becquerel • Uranium salt • 1898 Marie and Pierre Curie • 2 new elements: polonium and radium

17. Self check • 1). Describe the properties of the strong force • 2). Compare the strong force between protons and neutrons in a small nucleus and a large nucleus. • 3). Explain why large nuclei are unstable.

18. Section 2 Nuclear Decay • Why it’s important? • Nuclear decay produces nuclear radiation that can both harm people and be useful

19. What You Will Learn: • Compare and contrast alpha, beta, and gamma radiation • Define the half-life of a radioactive material • Describe the process of radioactive dating

20. Nuclear Radiation • Occurs when an unstable nucleus decays, particles and energy called nuclear radiation are emitted from it. • 3 types: • Aplha • Beta • Gamma

21. Alpha Particles (p 541) • Alpha particle: • Made of two protons and two neutrons is emitted from the decaying nucleus. • See Table 1 • Compared to beta and gamma radiation, alpha particles are much more massive • They have the most electric charge (therefore, lose energy more quickly when they interact with matter than the other types of nuclear radiation do).

22. Damage from Alpha Particles (p.542) • Can be dangerous if they are released by radioactive atoms inside the human body. • A single alpha particle can damage many fragile biological molecules. • Can cause cells not to function properly, leading to illness and disease. • Ex. Smoke detectors give off alpha particles that ionize the surrounding air.

23. Transmutation (p. 542) • The process of changing one element to another through nuclear decay • See Figure 8 on page 542

24. Beta Particles • When an electron is emitted from the nucleus • Beta decay is caused by another basic force called the weak force. • Damage from Beta Particles • Beta particles are much faster and more penetrating than alpha particles • They can pass through paper but are stopped by a sheet of aluminum foil • Damage cells when they are emitted by radioactive nuclei inside the human body

25. Gamma Rays • Electromagnetic waves with the highest frequencies and the shortest wavelengths in the electromagnetic spectrum. • Contain no mass and no charge • Travel at the speed of light • Emitted from a nucleus when alpha decay or beta decay occurs. • See Table 3 on Page 543 • What stops gamma rays? • Thick blocks of dense materials (lead and concrete) • They cause less damage to biological molecules as they pass through living tissue.

26. Radioactive Half-life • Half-life: The amount of time it takes for half the nuclei in a sample of the isotope to decay. • Radioactive dating: • Geologists, biologists, and archaeologists, among others, are interested in the ages of rocks and fossils found on Earth. • First: the amount of the radioactive isotope and its daughter nucleus in a sample of material are measured. • Second, the number of half-lives that need to pass to give the measured amounts of the isotope and its daughter nucleus is calculated. • Third, the number of half-lives is the amount of time that has passed since the isotope began to decay

27. Carbon Dating • Carbon-14 often is used to estimate the ages of plant and animal remains. • See page 545

28. Uranium Dating • Radioactive dating also can be used to estimate the ages of rocks. • Some rocks contain uranium, which has two radioactive isotopes with long half-lives.

29. Checks for understanding • 1. Infer how the mass number and the atomic number of a nucleus change when it emits a beta particle. • 2. Describe how each of the three types of radiation can be stopped.

30. Section 4: Nuclear Reactions (p 551) • What you will learn… • Explain nuclear fission and how it can begin a chain reaction • Discuss how nuclear fusion occurs in the Sun. • Describe how radioactive tracers can be used to diagnose medical problems. • Discuss how nuclear reactions can help treat cancer. VOCABULARY: nuclear fission, chain reaction, critical mass, nuclear fusion, and tracer.

31. Nuclear Fission (p. 551) • Nuclear fission: • The process of splitting a nucleus into several smaller nuclei • The word “fission” means to divide • Only large nuclei, such as the nuclei of uranium and plutonium atoms, can undergo nuclear fission. • Figure 16 • The products of a fission reaction usually include several individual neutrons in addition to the smaller nuclei

32. Chain Reactions/Critical Mass (p. 552) • Chain Reactions: • The series of repeated fission reactions caused by the release of neutrons in each reaction • If the chain reaction is uncontrolled, an enormous amount of energy is released in an instant. However, it can be controlled by adding materials that absorb neutrons. • Critical Mass: • The amount of material required so that each fission reaction produces approximately one more fission reaction. • If less than the critical mass of material is present, a chain reaction will not occur

33. Nuclear Fusion (p. 553) • Nuclear Fusion: • 2 nuclei with low masses are combined to form one nucleus of larger mass. • Fusion: fuses atomic nuclei together • Fission: splits nuclei apart • For Fusion to occur: positively charged nuclei must get close to each other. • Example: The Sun • Most of the energy given off by the Sun is produced by a process involving the fusion of hydrogen nuclei.

34. Tracer (p. 554) • Tracer: • A radioisotope that is used to find or keep track of molecules in an organism. • Scientist can use tracers to follow where a particular molecule goes in your body or to study how a particular organ functions. • Also used in agriculture to monitor the uptake of nutrients and fertilizers.

35. Treating Cancer with Radioactivity • Radiation can be used to stop some types of cancerous cells from growing. • Cancer: a group of cells in a person’s body grows out of control and can form a tumor

36. Summary: • Nuclear Fission • Occurs when a neutron strikes a nucleaus, causing it to split into smaller nuclei • A chain reaction requires a critical mass of fissionable material • Nuclear Fusion • Nuclear fusion occurs when two nuclei combine to form another nucleus • Nuclear fusion occurs at temperatures of millions of degrees which occur inside the Sun.

37. Check for understanding: • Explain how a chain reaction can be controlled. • Describe two properties of a tracer isotope used for monitoring the functioning of an organ in the body.