Ch19 Nuclear Reactions Harnessing the Power of the Sun Nuclear vs. Chemical Chemical changes involve the transfer of electrons. In chemical reactions atoms are re-arranged to form new products. Nuclear reactions involve a change in the nucleus of the atom. A change in protons or neutrons will create a new species, either an isotope or a new element. Notation Mass number is the total number of protons and neutrons. Atomic number is the total number of protons Nuclear changes are balanced using conservation of mass number and atomic number “Use the force Luke” What holds the net positive nucleus together? The strong nuclear force overcomes the electrostatic repulsion of the protons and binds the nucleus together. Nuclear Binding Energy Energy is required to bind the nucleus together. This energy comes from the difference in mass between the atom and its parts. Mass defect is the difference in mass between the number of neutrons and protons and the mass of the nucleus together. E = mc2 gives the amount of energy Mass Defect Proton Mass = 1.007825 Neutron mass = 1.008665 Mass of O-16 is 15.994915 Difference in mass between 8 protons and 8 neutrons is the mass defect (.137005). It is converted into energy to hold the nucleus together using e=mc2. Nuclear Decay Nuclear processes occur because of an instability between the number of protons and neutrons in the nucleus. During nuclear reactions, a particle from the nucleus is given off which results in: Change of proton to a neutron Change of neutron to a proton Release of energy and radiation Valley of Stability indicates stable nuclei. The ratio of neutrons to protons increases for stable nuclei as mass number increases. Nuclear Decay Particles Alpha particle A particle with 2 protons and 2 neutrons Alpha particles are emitted The net effect is to lose 4 nuclei particles from the nucleus The daughter particle is the element two elements back on the periodic table. Nuclear Decay Particles Beta particle ß Electron emitted from the nucleus Changes a neutron to a proton The daughter formed is the next element on the periodic table Nuclear Decay Particles Positron emission ß+ Positive electron emitted from the nucleus Changes a proton to a neutron The daughter formed is the element directly behind the parent nuclei. Nuclear Decay Particles Gamma particle A high energy particle of radiation, no mass The particle is given off as the nucleus returns to a lower energy state The parent and daughter are the same Nuclear Decay Particles Electron Capture Turns a proton to a neutron by capturing an electron The same process as positron emission The daughter formed is the element preceding the parent nucleus. Radioactive equations Atomic Fission Nuclear fission occurs when a heavy nucleus such as U-235 splits into two smaller nuclei. The total rest mass of the products is less than the original rest mass. The mass difference is converted to the energy released using e=mc2 This process occurs naturally at a slow rate, but can be accelerated by neutron bombardment in the laboratory. Neutron Bombardment of U-235 U n U 235 92 1 0 236 92 U X Y neutrons 236 92 U Ba Kr 3 n 236 92 141 56 92 36 1 0 U-235 absorbs a slow moving neutron. U-236 is unstable and lasts for only 10-12 s. The U-236 nucleus splits into two fragments and emits several neutrons. Nuclear Fusion Lighter nuclei undergo a process called nuclear fusion Fusion takes a great amount of energy to be initiated in order to overcome the proton-proton barrier between nuclei. However, a greater amount of energy is released in the process. Stars produce their energy through the fusion process. Fusion Process 1 1 H H H e H H He 1 1 1 1 1 1 2 1 2 1 0 1 3 2 H He He e 3 2 4 2 0 1 3 2 He He He H H 3 2 4 2 1 1 1 1 Hydrogen is fused together forming deuterium. He-3 is formed from hydrogen isotopes. He-4 is formed through 2 different processes. The energy is produced though the gamma, neutrino, and positron emission.