Radioactivity and Nuclear Reactions

Transcription

1 chapter 20 Radioactivity and Nuclear Reactions section 1 The Nucleus What You ll Learn what particles make up an atom and its nucleus how the nucleus is held together what radioactivity is Before You Read Have you ever heard of an object, such as a chair, being described as unstable? In the space below, give examples of an object that is unstable and an object that is stable. Study Coach Make Flash Cards Make flash cards of important terms you read about in this section. Write term on one side of a flash card and its definition on the other side. Keep reviewing the flash cards until you know all the terms and their definitions. 1. Identify What type of charge does an electron have? Read to Learn Describing the Nucleus Even though you may not know it, particles are constantly hitting you. Most of these particles are from stable atoms. But some of these particles are from unstable atoms in soil, rocks, and the air. Which atoms are unstable? What particles do they emit, or give off? The answers to these questions begin with the nucleus of an atom. What are protons and neutrons? Recall that an atom is made up of very small particles. The particles are protons, neutrons, and electrons. The protons and neutrons are located in the nucleus at the center of an atom. Protons are positively charged particles. Neutrons have no electric charge. Neutrons are electrically neutral. Since the nucleus contains positively charged protons, it has a positive charge. Each proton has one positive electrical charge, or 1. The total amount of positive charge in a nucleus is equal to the number of protons that the nucleus has. The number of protons in a nucleus is called its atomic number. Atoms usually contain the same number of protons as electrons. Electrons have a negative charge. Electrons are attracted to the positively charged nucleus. This electric attraction pulls the electrons close to the nucleus. 358 CHAPTER 20 Radioactivity and Nuclear Reactions

2 Is the nucleus the largest part of an atom? Protons and neutrons are packed together tightly in the nucleus of an atom. The particles in the nucleus are so close that the nucleus takes up only a tiny part of an atom. The part outside the nucleus in which the electrons are located is much larger than the nucleus. Think of an atom as a football stadium. Its nucleus would be the size of a marble. Although the nucleus takes up very little space, it contains almost all the mass of an atom. What are isotopes? Look at the two helium nuclei in the figure below. How are they different? They both have two protons, but the one on the right has one more neutron. All atoms of the same element have the same number of protons in the nucleus. However, all the atoms of the same element do not always have exactly the same number of neutrons. Isotopes are atoms of the same element that have different numbers of neutrons. The atoms of all isotopes of the same element do have the same number of protons and electrons and the same chemical properties. A Find Main Ideas Make a Foldable like the one shown below. As you read this section, write down main ideas about the strong force. The Strong Force Picture This 2. Compare and Contrast How are the two helium isotopes different? Helium-3 Helium-4 How is a nucleus numbered? You can describe a nucleus using its atomic number. The atomic number of an atom is the number of protons in the nucleus. You can also describe a nucleus using its atomic mass. The nucleus contains almost all the mass of an atom, so the mass number of an atom is the number of protons and neutrons in the nucleus. Look at the diagram of helium-3 above. Its mass number is 3 because it contains a total of three protons and neutrons. But its atomic number is 2 because it contains two protons. Look at the diagram of helium-4 above. Its mass number is 4 because it contains a total of four protons and neutrons. However, the atomic number is 2 because the helium nucleus contains two protons. Reading Essentials 359

3 Picture This 3. Interpret Scientific Illustrations In the figure, circle the example showing that the strong force is greater than the electric force. The Strong Force Particles with the same charges repel, or push away from, each other. Why don t the positively charged protons in the nucleus repel each other? Another force holds the particles of the nucleus together. The strong force is the force that makes protons and neutrons attract each other and stay together. The strong force is one of the four basic forces in nature. The strong force is 100 times stronger than the electric force, but it only works when particles are close together. The figure below shows the strong force and the electric force. The two particles on the left are close, like the protons and neutrons in a nucleus, so the strong force is working. But the particles on the right are farther apart. So the strong force weakens, and the electric force takes over. So when protons are far apart, they are repelled by the electric force. Strong force No strong force 4. Describe Name the force that causes protons to push away from each other. How do forces work in a small nucleus? Not all nuclei (singular, nucleus) are the same size. Small nuclei have only a few protons and neutrons. The strong force holding the particles together is greater than the electric force that is pushing the particles apart. As a result, the protons and neutrons are held tightly together. How do forces work in a large nucleus? Even though there are more particles in a large nucleus than in a small nucleus, the strong force in a large nucleus is about the same strength as the strong force in a small nucleus. The strong force holds together particles that are closest together. In a nucleus with many protons, the electric force repels protons that are far apart. The diagram on the next page shows how the electric force that pushes the protons apart is greater in a large nucleus than in a small nucleus. The increased repulsive force causes the particles in a large nucleus to be held together less tightly than those in a small nucleus. 360 CHAPTER 20 Radioactivity and Nuclear Reactions

4 Strong force Strong force Picture This 5. Identify Circle the nucleus that has the larger repulsive force. Radioactivity When the strong force can hold a nucleus together forever, the nucleus is stable. If the strong force is not large enough to hold the nucleus together, the nucleus becomes unstable and can break apart or decay. When a nucleus decays, it emits particles and energy. Radioactivity is the process of a nucleus decaying and emitting particles and energy. Large nuclei are more unstable than small nuclei. All nuclei with more than 83 protons are radioactive. Some smaller nuclei are also radioactive. Even a nucleus with only one proton could be radioactive. Who discovered radioactivity? In 1896, Henri Becquerel made an interesting discovery. He left uranium salt in a drawer with a photographic plate. When he developed the plate, he saw an outline of the uranium salt on it. Becquerel realized that the uranium must have given off rays, or radiation, that darkened the film. Two years after Henri Becquerel s discovery of radioactivity, there was another important discovery. Marie and Pierre Curie discovered two new elements. The elements were polonium and radium. Both new elements were radioactive. Marie and Pierre Curie wanted to get a large sample of radium so they could study it. It took them more than three years to obtain about 0.1 g of radium from several tons of the mineral pitchblende. 6. Recall Who discovered the elements polonium and radium? Reading Essentials 361

5 After You Read Mini Glossary radioactivity: the process of a nucleus decaying and emitting particles and energy strong force: the force that makes protons and neutrons in the nucleus attract each other and stay together 1. Review the terms and their definitions in the Mini Glossary. Write a sentence on the lines below that shows your understanding of the term radioactivity. 2. Complete the table below to organize information about stable and unstable nuclei. Type of Nucleus Comparison of Radioactive? Example Strong Force and Electric Force Stable Unstable 3. Study Coach Think about what you have learned. How did making flash cards of important terms help you learn the content? End of Section 362 CHAPTER 20 Radioactivity and Nuclear Reactions