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Transcript

Nuclear Stability Notes “Why do protons stay together when positive charges repel each other?” The main reason is because of a force called Strong Force. Opposes the electrostatic force. 1 Nuclear Stability Notes The Strong Force is exerted by anything with mass (protons and neutrons) to attract other masses together and works within a very short distance. Neutrons act as insulation, since they have no charge, but have the strong force to bring other nucleons (protons and neutrons) together. 2 Nuclear Stability Notes As a general rule, a nucleus will need a neutron/proton ratio of 3:2 (or 1.5:1) in order to stay together. This rule is more precise for larger nuclei. 3 Nuclear Stability Notes Of all known isotopes of natural elements (about 1500), only 250 of them are stable. All of these stable isotopes have an atomic number in between 1 and 83. 4 Nuclear Stability Notes The amount of energy that keeps a nucleus together is called the Binding Energy. This amount of energy is higher for nuclei that are stable than it would be for unstable nuclei. It is measured in units called “Joules” or J. 5 Nuclear Stability Notes The amount of binding energy for a nucleus can be calculated using E = mc2. This equation states that mass can actually be converted into energy (during nuclear reactions). 6 Nuclear Stability Notes The mass of a nucleus will be less than the mass of all of the protons and neutrons making it up. The difference is called the mass defect, which is converted into energy if the nucleus is broken up. 7 Nuclear Stability Notes If the mass that changes is very small, will it release much energy? Energy = (change in mass-called mass defect x (speed of light)2 8 Nuclear Stability Notes Binding Energy Calculations: Need to know: speed of light = 3.00 x 108 m/s (speed of light)2 = 9.00 x 1016 m/s conversion factor for amu to kg = 1.66054 x 10-27 kg/amu mass of H atom (close to mass of proton)= 1.007825 amu mass of neutron = 1.008665 amu 9 Nuclear Stability Notes Binding Energy Calculation Examples: #1 (Teacher example)—calculate the binding energy of one atom of deuterium atoms (nucleus is composed of one proton and one neutron; measure atomic mass is 2.0140 amu). 10 Nuclear Stability Notes Binding Energy Calculation Examples: #2 (Class practice)—Calculate the binding energy of one lithium-6 atom whose nucleus contains three protons and three neutrons; measured atomic mass of lithium-6 is 6.0151 amu. 11