Chemistry: Nuclear Reactions Guided Inquiry

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1 Chemistry: Nuclear Reactions Guided Inquiry Nuclear reactions change the nucleus of an atom. Chemical Reactions vs. Nuclear Reactions Atoms and molecules are striving to achieve the most stable arrangement. Chemical reactions involve the breaking of chemical bonds in the original compounds (reactants), the rearranging of the atoms and the formation of new chemical bonds to create different compounds (products). While chemical reactions create new compounds, the atoms are not changed. The number and type of each atom present at the beginning of the reaction will be present at the end of the reaction. Chemical reactions obey the Law of Conservation of Mass and the Law of Conservation of Energy. The reaction of hydrogen and oxygen to produce water shows how the at atoms are rearranged, but the atoms themselves remain unchanged. We start the reaction with 4 hydrogen atoms (in blue) and 2 oxygen atoms (in red) and at the end of the reaction we still have 4 hydrogen atoms and 2 oxygen atoms. Nuclear reactions are reactions that affect the nucleus of an atom. In nature, unstable nuclei undergo nuclear reactions to form more stable nuclei. Stable nuclei can also undergo nuclear reactions if they are bombarded with subatomic particles at high speed. This last process is an artificial, man- made process. In all of these reactions a new isotope or nuclide is formed and particles and/or electromagnetic waves are emitted. In some instances, the nuclide is a new element. Unlike chemical reactions, the atoms are changed in nuclear reactions. Also, mass and energy are not conserved. Rather they are interchangeable according to Einstein s famous equation E = mc 2. That is why nuclear reactions either require or release large amounts of energy. To the right is an example of one of the nuclear reactions that occur in the Three Mile Island nuclear reactor. In this reaction, a neutron collides with a uranium- 235 atom. The atom splits into two smaller atoms, barium- 144 and krypton- 89, while emitting 3 neutrons, gamma rays, and a vast amount of thermal energy. We use nuclear equations to document the changes that occur during a nuclear reaction. The nuclear equation for the above reaction is # " U + n Ba + Kr + 3 n + γ As we learned earlier in the year, when using atomic (aka isotope) symbols to represent an atom, the mass number is put in the upper left hand corner and the atomic number is put in the lower left hand corner. Notice that both the mass number and atomic number are conserved in the above reaction. The mass numbers of all the reactants on the left hand side of the arrow add up to 236 ( ), and the mass numbers of all the products add up to 236 ( x1). The atomic numbers of all the reactants add up to 92 (92 + 0), and the atomic numbers of all the products add up to 92 ( x0). While mass and energy are not conserved in nuclear reactions, mass number and atomic number are conserved. This is an important concept in understanding nuclear reactions.

2 The atomic symbols for the 4 types of nuclear radiation are: alpha particle - He beta particle - β positron - β gamma ray - γ 1. Polonium- 211 spontaneously decays to form lead- 207 and an alpha particle. Write the nuclear equation to represent this nuclear reaction. STOP Show your nuclear equation to the teacher. Types of Nuclear Reactions There are 4 general types of nuclear reactions: radioactive decay, fission, fusion and artificial transmutation. Radioactive decay is the spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by the emission of particles and/or electromagnetic radiation. We will study 5 types of decay reactions. Alpha Decay occurs in heavy nuclei that need to lose both protons and neutrons to become stable. An alpha particle is a nucleon with 2 protons, 2 neutrons and a 2+ charge. It is a helium atom with its electrons stripped away. The nucleon s atomic number decreases by 2 and its mass number decreases by 4. Example Write the nuclear equation for the alpha decay of uranium # # He + Th U 2. Write the nuclear equation for the alpha decay of plutonium Beta Decay occurs in nuclei that need to lose neutrons to become stable. In this decay a neutron is converted into a proton and an electron. A beta particle is an electron that is emitted during this decay. The nucleon s atomic number increases by 1 and its atomic mass stays the same. Example Write the nuclear equation for the beta decay of uranium # # β + Np U 3. Write the nuclear equation for the beta decay of plutonium Positron Decay - occurs in nuclei that need to lose protons to become stable. In this decay a proton is converted into a neutron, a positron, and a neutrino (ν). We will ignore the neutrino when writing equations because it has no mass number or atomic number. A positron is a particle that has the mass of an electron, but the charge of a proton. In positron decay, the nucleon s atomic number decreases by 1 and its atomic mass stays the same. Example Write the nuclear equation for the positron decay of magnesium- 23. Mg β + Na 4. Write the nuclear equation for the positron decay of bromine- 75.

3 Electron Capture Decay occurs in nuclei that need to lose protons to become stable. In this decay an electron is captured by the nucleus and combined with a proton to form a neutron. The nucleon s atomic number decreases by 1 and its atomic mass stays the same. Example Write the nuclear equation for the electron capture decay of cobalt- 56. Co + e Fe 5. Write the nuclear equation for the electron capture decay of mendelevium Gamma Emission usually occurs immediately following other types of decay that leave the nucleus in a temporarily excited (higher energy) state. When the nucleus returns to its ground (lower energy) state the loss in energy is emitted as gamma rays. Gamma rays are high- energy electromagnetic waves that travel at the speed of light. They have no mass and therefore are not matter (particles). STOP Show your nuclear equations to the teacher. A decay series is a series of nuclear reactions that occur to convert an unstable nucleon into a stable nucleon. The final nucleon is stable, but the intermediate nucleons formed during this series of reactions are unstable. Uranium- 238, which is unstable, undergoes a series of alpha and beta decay reactions that eventually produce a stable lead- 206 atom. Some of these decay reactions happen in milliseconds, while the initial alpha decay reaction takes billions of years. 6. The order of the first 4 reactions in the uranium series are: alpha decay, beta decay, beta decay and alpha decay. Write these first 4 decay reactions. STOP Show your nuclear equations to the teacher.

4 Nuclear fission and nuclear fusion reactions generate vast quantities of energy, but there are some significant differences between the two reactions. General Description Fission A heavy nucleus is split in to more stable nuclei that are lighter in mass. Fusion Light- mass nuclei are combined to form a heavier, more stable nucleus. # " n Examples U + n Ba + Kr + 3 # # U + n Br + Kr + 3 n # # " Pu + n Mo + Te + 4 n 4 H 2 β + He H + H n + He 2 He 2 H + He Conditions for reaction to occur critical mass of the substance and a fast moving neutron a high density, high temperature environment Reactant(s) heavy nucleus + neutron light nuclei such as isotopes of hydrogen and helium Product(s) Uses/Where it Occurs Controllable Reaction Chain Reaction lighter nuclei + neutrons + highly radioactive particles + large amounts of energy a million times more energy than a chemical reaction Does not occur naturally. Man- made reactions are used in nuclear power plants to generate electricity, and in atomic bombs. Yes, control rods in the nuclear reactors are used to absorb the neutrons. Yes, provided there is enough material. This amount is known the critical mass. heavier nuclei + large amounts of energy * no radioactive particles 3x to 4x the energy of a fission reaction Occurs naturally in stars like our sun. Man- made reactions are used in hydrogen bombs. No, but hopefully in the future. Yes, provided there are extremely high temperatures (thousands of degrees Celsius). Chain reactions are reactions in which one of the materials that start the reaction (aka reactant) is also one of the products and can start another reaction. In an uncontrolled chain reaction the amount of reactant produced in the first reaction is enough to start two or more additional reactions and subsequent reactions occur exponentially faster. Both conventional and nuclear bombs are examples of uncontrolled chain reactions. 7. Why is nuclear fission a chain reaction? 8. Why is nuclear fusion a chain reaction?

5 Artificial transmutation is the process where stable nuclei are bombarded with charged and uncharged particles to create artificial (man- made), radioactive nuclides (elements). In some instances the nuclei are bombarded with neutrons, and in other instances they are bombarded with larger particles like alpha particles and carbon nuclei. These bombarding particles must collide with the nuclei at very high speeds. Particle accelerators use powerful electromagnetic charges to accelerate the particles for many kilometers before they collide with the nuclei. Fermilab s Tevatron shown below is 3.9 miles long. Diagram of particle accelerator at Fermilab. Arial view of particle accelerator at Fermilab. Transmutation has been used to create radioactive isotopes of all naturally occurring elements. Uranium, which has an atomic number of 92, is the largest naturally occurring nucleon. Transmutation has also been used to create artificial nuclides with more than 92 protons. These large, man- made elements are called trans- uranium elements. Below are the reactions for creating some of the trans- uranium elements. Atomic # Name Symbol Nuclear Reaction # 93 neptunium Np U + n # # # 94 plutonium Pu Np Pu + β # U Np + β # 96 curium Cm Pu + He Cm + n # # 102 nobelium No Cm + C No + 4 n Artificial transmutation is also used to make radioactive isotopes (aka radioisotopes) of lighter elements such as cobalt, iron, copper, calcium, cesium, iodine, gallium, phosphorous, and many others. Several of these radioisotopes are used in medicine to research, diagnose and treat illnesses and genetic defects. Others are used in industry. 9. How are artificial elements made? # STOP - Take Nuclear Reactions Quick Quiz