Radioactivity and Nuclear Reactions Chapter 18 Physical Science

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

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

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

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

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

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

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

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)

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#

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

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

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.

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.

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.

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

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.

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

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

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

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).

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.

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

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

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.

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

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

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.

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.

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.

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

Chain Reactions/Critical Mass (p. 552) 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

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.

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.

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

Summary: Nuclear Fission Nuclear Fusion 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.

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.