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chapter29 - Chapter 29 Nuclear Physics Ernest Rutherford...

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Chapter 29 Nuclear Physics
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Ernest Rutherford 1871 1937 Discovery that atoms could be broken apart Studied radioactivity Nobel prize in 1908
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Some Properties of Nuclei All nuclei are composed of protons and neutrons Exception is ordinary hydrogen with just a proton The atomic number , Z, equals the number of protons in the nucleus The neutron number , N, is the number of neutrons in the nucleus The mass number , A, is the number of nucleons in the nucleus A = Z + N Nucleon is a generic term used to refer to either a proton or a neutron The mass number is not the same as the mass
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Symbolism Symbol: X is the chemical symbol of the element Example: Mass number is 27 Atomic number is 13 Contains 13 protons Contains 14 (27 13) neutrons The Z may be omitted since the element can be used to determine Z X A Z Al 27 13
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More Properties The nuclei of all atoms of a particular element must contain the same number of protons They may contain varying numbers of neutrons Isotopes of an element have the same Z but differing N and A values Example: 11 12 13 14 6 6 6 6 C C C C
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Charge The proton has a single positive charge, +e The electron has a single negative charge, -e The neutron has no charge Makes it difficult to detect e = 1.602 177 33 x 10 -19 C
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Mass It is convenient to use unified mass units, u, to express masses 1 u = 1.660 559 x 10 -27 kg Based on definition that the mass of one atom of C-12 is exactly 12 u Mass can also be expressed in MeV/c 2 From E R = m c 2 1 u = 931.494 MeV/c 2
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Summary of Masses
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The Size of the Nucleus First investigated by Rutherford in scattering experiments He found an expression for how close an alpha particle moving toward the nucleus can come before being turned around by the Coulomb force The KE of the particle must be completely converted to PE
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Size of the Nucleus, cont d gives an upper limit for the size of the nucleus Rutherford determined that For gold, he found d = 3.2 x 10 -14 m For silver, he found d = 2 x 10 -14 m Such small lengths are often expressed in femtometers where 1 fm = 10 -15 m Also called a fermi 2 2 4 e k Ze d mv
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Size of Nucleus, Current Since the time of Rutherford, many other experiments have concluded: Most nuclei are approximately spherical Average radius is r o = 1.2 x 10 -15 m A is the number of nucleons 1 3 o r r A
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Density of Nuclei The volume of the nucleus (assumed to be spherical) is directly proportional to the total number of nucleons This suggests that all nuclei have nearly the same density Nucleons combine to form a nucleus as though they were tightly packed spheres
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Maria Goeppert-Mayer 1906 1972 Best known for her development of shell model of the nucleus Shared Nobel Prize in 1963
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Nuclear Stability There are very large repulsive electrostatic forces between protons These forces should cause the nucleus to fly apart The nuclei are stable because of the
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