CHAPTER 0 2 SECTION Nuclear Changes Nuclear Fission and Fusion KEY IDEAS As you read this section, keep these questions in mind: What holds the nucleus of an atom together? What happens when the nucleus of a heavy atom splits apart? What happens when the nucleus of a small atom joins with the nucleus of another small atom? Who Discovered Nuclear Fission? In 939, German scientists Otto Hahn and Fritz Strassman were trying make uranium atoms with heavier nuclei. To do this, they used a device that shot neutrons at a sample of uranium. They hoped that at least a few uranium nuclei would gain one or more neutrons. To their surprise, new elements formed. Instead of uranium isotopes, they detected barium and krypton. They thought they had made a mistake. Soon after, a scientist named Lise Meitner and her nephew Otto Frisch read the results of the experiments. Meitner realized that Hahn and Strassman had not made a mistake. They had actually split the uranium nuclei into smaller elements. READING TOOLBOX Define As you read, write down any terms in the section that you do not understand. Use the section vocabulary boxes or a dictionary to find the definitions of these terms. Write the definitions in your own words in the text margin.. Explain What did Lise Meitner conclude about the work of Hahn and Strassman? This nuclear reactor was used in the late 940s and early 950s to study controlled nuclear fission. In the early 940s, Enrico Fermi and other scientists at the University of Chicago built a stack of graphite and uranium blocks. These materials formed the nuclear reactor that was used to create the first controlled nuclear fission chain reaction. This work launched the Manhattan Project, which led to the creation of nuclear weapons. Interactive Reader 23 Nuclear Changes
What Holds a Nucleus Together? Protons and neutrons are packed tightly in the tiny nucleus of an atom. Recall that some atomic nuclei are unstable and emit nuclear radiation as they decay. The stability of a nucleus depends on the nuclear forces that hold the nucleus together. If like charges repel one another, how can positively charged protons fit together into an atomic nucleus without flying apart? 2. Compare Compare the relative strengths of the nuclear force and the electric force. THE STRONG NUCLEAR FORCE The strong nuclear force holds the neutrons and protons together in a nucleus. This attraction is much stronger than the electric force that causes protons to repel other protons. However, the attraction only happens over a very short distance (3 0 5 meters). This is less than the width of three protons. The strong nuclear force that holds protons and neutrons together is stronger than the electrical force that pushes protons apart. 3. Describe What are the relative strengths of attractive and repulsive forces in a stable nucleus? THE STRONG NUCLEAR FORCE AND STABILITY Because neutrons have no charge, they do not attract or repel protons or one another. However, protons repel one another due to the electric force and attract one another due to the nuclear force. In stable nuclei, the attractive forces are stronger than the repulsive forces. Under these conditions, the element does not undergo decay. Interactive Reader 24 Nuclear Changes
What Causes a Nucleus to Decay? An element decays when the repulsive forces in the nucleus are stronger than the attractive forces. A large number of neutrons in an atom can help hold a nucleus together. However, there is a limit to how many neutrons a nucleus can have. Nuclei with too many or too few neutrons are unstable and undergo decay. UNSTABLE NUCLEI Nuclei with more than 83 protons are always unstable, no matter how many neutrons they have. These nuclei always decay and release large amounts of energy and nuclear radiation. Some of the energy is passed to the particles that leave the nucleus. The rest of the energy leaves the nucleus in the form of gamma rays. This radioactive decay helps produce a more stable nucleus. What Is Nuclear Fission? The process of splitting atoms with heavy nuclei into atoms with lighter nuclei is called fission. When the nucleus splits, both neutrons and energy are released. In their experiments, Hahn and Strassman used a device that shot at, or bombarded, a uranium- nucleus with neutrons. One set of products from this type of fission includes two lighter nuclei, barium-40 and krypton-93, and neutrons and energy. U + 92 n 0 40 93 56 Ba + Kr + 3 36 n + energy 0 Notice that the products include three neutrons plus energy. Uranium- can also undergo fission to produce different pairs of lighter nuclei. For example, uranium- can undergo fission to produce strontium-90, xenon-43, and three neutrons. How Does Nuclear Fission Produce Energy? During fission, the nucleus breaks into smaller nuclei. The process releases large amounts of energy. Each dividing nucleus releases about 3.2 0 joules of energy. In comparison, the chemical reaction of one molecule of the explosive trinitrotoluene (TNT) releases 4.8 0 8 joules. 4. Identify Under what conditions does a nucleus decay? 5. Identify What is the maximum number of protons that can be found in a stable nucleus? 6. Analyze How many total neutrons are on the left side of the equation? How many total neutrons are found on the right? Math Skills 7. Compare How much more energy is released by a dividing nucleus than by a molecule of TNT? Interactive Reader 25 Nuclear Changes
When a uranium- nucleus is bombarded by a neutron, the nucleus breaks apart into smaller nuclei. The process releases energy through fast moving neutrons. 8. Explain Why is some mass missing after fission? 9. Explain What does massenergy equivalence mean? In their experiment, Hahn and Strassman recorded the masses of all of the nuclei and particles before and after the reaction. They found the overall mass had decreased after the reaction. Hahn and Strassman also found that the process had released energy. They concluded that the missing mass must have changed into energy. What Is Mass-Energy Equivalence? Recall that, according to Newton s laws, no mass or energy can be created or destroyed during physical or chemical changes. The laws of conservation of mass and energy do not apply to nuclear reactions such as fission. During fission, some matter changes to energy. Albert Einstein explained the equivalence of mass and energy by the special theory of relativity. This equivalence means that matter can be converted into energy, and energy can be converted into matter. Equivalence is described by the equation below. Mass-Energy Equation Energy = mass (speed of light) 2 E = mc 2 The speed of light, c, equals 3.0 0 8 m/s. If you multiply c 2 by even a very small mass, the energy value is very large. For example, the mass-equivalent energy of kg of matter is 9 0 6 joules. That is more than the energy of 22 million tons of the explosive TNT. Interactive Reader 26 Nuclear Changes
STABILITY OF MATTER Obviously, the objects around us do not change suddenly into their equivalent energies. The results of that would be disastrous. Under ordinary conditions, matter is very stable. What Is the Mass Defect? Suppose you measured the mass of a carbon-4 nucleus, which has six protons and eight neutrons. Suppose you then measured the mass of six individual protons and eight individual neutrons. You would expect the mass of the nucleus to be the same as the total mass of the individual particles. However, you find that the mass of the nucleus is less than the sum of the individual masses. What happened to the missing mass? Mass of proton =.673 0 27 kg Mass of 6 protons = Mass of neutron =.675 0 27 kg Actual mass of a carbon-4 nucleus = 2.325 0 26 kg Mass of 8 neutrons = Expected mass of a carbon-4 nucleus = Math Skills 0. Calculate Complete the table to compare the expected and actual values for the mass of a carbon-4 nucleus. The missing mass is called the mass defect. Einstein s theory of special relativity explains: the missing mass changes into energy. When nuclei form, energy is released. Note, however, that the mass defect of a nucleus is very small. What Is a Nuclear Chain Reaction? Have you ever watched balls moving on a pool table? When one ball hits another, the collision can cause the second ball to hit another. Some nuclear reactions work the same way. One reaction triggers another reaction. A nucleus that splits when it is hit by a neutron forms smaller nuclei. The smaller nuclei need fewer neutrons to be held together. Therefore, they release extra neutrons. If one of those neutrons collides with another large nucleus, that nucleus undergoes fission, or splits. A nuclear chain reaction is a continuous series of nuclear fission reactions.. Define What is the mass defect? Interactive Reader 27 Nuclear Changes
2. Predict How many total neutrons will probably be released in the next stage of this chain reaction? 93 36 Kr 40 56 Ba 93 36 Kr 40 56 Ba 40 56 Ba 93 36 Kr 40 56 Ba 93 36 Kr A nuclear chain reaction may be triggered, or started, by a single neutron. Scientists found that on average, each uranium nucleus that divides produces two or three extra neutrons. Each one of these neutrons could trigger, or start, another fission reaction. The ability to start a nuclear chain reaction depends partly on the number of neutrons released during each fission reaction. What Is a Controlled Chain Reaction? Energy produced in a nuclear chain reaction can be used to generate electricity. The diagram describes this process. Using Nuclear Chain Reactions to Generate Electricity 2 Uranium- nuclei in the fuel rod (black) undergo a chain reaction. Control rods (gray) absorb neutrons. This keeps the chain reaction at a safe level. A coolant, usually water, absorbs energy from the chain reaction. 3 Water absorbs energy from the hot coolant and changes to steam. 3. Identify What is the function of the control rods? To cooling tower 4 The steam turns a turbine attached to a generator. 5 The generator changes the mechanical energy of the spinning turbine into electrical energy. Interactive Reader 28 Nuclear Changes
NUCLEAR WEAPONS The chain-reaction principle is also used to make a nuclear bomb. In a nuclear bomb, two or more quantities of uranium- are packed into a container. The uranium is surrounded by a powerful chemical explosive. When the explosives are detonated, or set off, the uranium is pushed together to exceed the critical mass. The critical mass is the smallest amount of a substance that provides enough neutrons to start a nuclear chain reaction. If the amount of a substance is less than the critical mass, a chain reaction will not continue. Fortunately, the concentration of uranium- in nature is too low to start a chain reaction naturally. In nuclear power plants, control rods are used to slow the chain reaction. In a nuclear bomb, reactions are not controlled. What Is Nuclear Fusion? Nuclear fission is not the only nuclear process that can produce energy. Energy can also be produced when light atomic nuclei join, or fuse, to form heavier nuclei. This process is called fusion. In the sun and other stars, huge amounts of energy are produced when hydrogen nuclei fuse. However, a large amount of energy is needed to start a fusion reaction. Energy is needed to push nuclei close enough so that the strong nuclear force can overcome the repulsive electrical force. In stars, extremely high temperatures provide the energy to bring hydrogen nuclei together. 4. Define What is a critical mass? 5. Describe What happens during nuclear fusion? H+ H 2 H + other particles 2 H+ H 3 2 He + 0 0 γ 3 2 He + 3 2 He 4 2 He+ H+ H The process of nuclear fusion releases large amounts of energy. Interactive Reader 29 Nuclear Changes
Section 2 Review SECTION VOCABULARY critical mass the minimum mass of a fissionable isotope that provides the number of neutrons needed to sustain a chain reaction fission the process by which a nucleus splits into two or more fragments and releases neutrons and energy fusion the process in which light nuclei combine at extremely high temperatures, forming heavier nuclei and releasing energy nuclear chain reaction a continuous series of nuclear fission reactions. Summarize Complete the process chart to describe how nuclear fission is used to produce electricity. A coolant absorbs energy from the chain reaction. A generator changes the mechanical energy of the spinning turbine into electrical energy. 2. Predict Suppose you had an atom of 56 26 Fe. Is the mass of its nucleus greater than, less than, or equal to the combined masses of 26 protons and 3eutrons? Explain your answer. 3. Identify Do the following equations describe nuclear fission or nuclear fusion? Explain your answers. U+ 92 n 0 40 93 56 Ba + Kr + 3 36 n + energy 0 208 82 58 265 Pb + 26 Fe Hs + 08 n 0 Interactive Reader 220 Nuclear Changes