Radioactive Decay Nuclear reactions are written in a format similar to chemical reactions Reactants and products are atoms or subatomic particles instead of molecules Nuclide symbols E are written to represent the composition of a nuclide These symbols can be used to represent atoms ions and nuclei 1 Radioactive Decay Nuclide symbols for subatomic particles Atomic number charge on the nucleus Nuclear reactions are written using nuclide symbols Nuclear reactions are balanced when the sums of the mass numbers and atomic numbers for both sides of the equation are equal 2 Radioactive Decay A thin sheet of aluminum blocks alpha rays but not beta rays In a magnetic field beta and alpha particles are deflected in different directions while gamma rays are undeflected 3 Radioactive Decay Alpha particles are the more massive and positively charged particles Alpha particles are helium nuclei Beta particles are lighter and negatively charged Beta particles are electrons emitted from the nucleus Gamma rays are the particles unaffected by the magnetic field Gamma rays are high energy photons of electromagnetic radiation emitted by the nucleus 4 Alpha Decay During alpha decay an alpha particle is emitted from the nucleus The mass number decreases by 4 The atomic number decreases by 2 The reactant nucleus is the parent The product nucleus is the daughter 5 Beta Decay During beta decay a beta particle and an antineutrino emitted from the nucleus are A neutron decays into a proton a beta particle and an antineutrino The proton remains in the nucleus The atomic number increases by 1 6 Gamma Decay Gamma decay is the emission of high energy photons and tends to accompany other types of decay Protons and neutrons occupy energy levels within a nucleus analogous to energy levels for electrons When alpha and beta particles leave the nucleus the nucleus is left in an excited state The nucleus returns to the ground state by emitting a gamma ray 7 Gamma Decay The energy level spacing in the nucleus is very large The emitted gamma rays have high energies Wavelength 10 12 m and frequency 3 1020 s 1 Gamma ray energies are on the order of 108 kJ mol several orders of magnitude larger than ordinary chemical reaction energies Gamma decay does not change the atomic number or mass number of a nucleus and generally accompanies beta decay 8 Electron Capture and Positron Emission In electron capture the nucleus captures an electron converting a proton to a neutron and decreases the nuclear charge by one The reverse of beta emission Positron decay occurs when a proton decays into a neutron positron and a neutrino A positron is a positively charged electron The nuclear charge decreases by one 9 Positron Emission A positron and an electron form a matter antimatter pair Matter antimatter pairs are identical in mass and spin but opposite in charge When a positron and electron collide they are annihilated and their mass converted into energy Positron electron collisions produce two 511 keV gamma ray photons traveling in opposite directions 10 Kinetics of Radioactive Decay The activity of a sample of N nuclei is the rate of disintegration of the sample given by N t The SI unit of nuclear activity is the becquerel Bq defined as one nuclear disintegration per second dps The curie Ci is an older and larger unit defined as the number of disintegrations per second in 1 gram of radium 226 1 Ci 3 7 1010 Bq 11 Kinetics of Radioactive Decay For radioactive decay the activity of a sample decreases exponentially with time The activity is proportional to the number of nuclei present N N also decreases exponentially N0 is the initial number of nuclei and k is the decay constant Radioactive decay follows first order kinetics 12 Kinetics of Radioactive Decay The half life t1 2 is the time required for half the sample to disintegrate Radioactive decay always follows first order kinetics The half life is constant for any given isotope 13 Radiocarbon Dating Half lives of some radioactive isotopes Long lived isotopes such as uranium can be used to date minerals and geological formations 14 Nuclear Stability The Chart of the Nuclides is a plot of the number of protons versus the number of neutrons for all known stable nuclei The chart is used in a manner similar to the periodic table to look for patterns and trends to explain nuclear stability Virtually all stable nuclides are found in the central region in the chart of the nuclides in a region called the band of stability The nuclides outside the band of stability are in the region referred to as the sea of instability 15 Nuclear Stability The chart of the nuclides is a plot of atomic number Z versus neutron number N for all known nuclides All stable isotopes lie in the region shown with blue dots 16 Nuclear Stability The decay series starting with 238U involves a series of alpha and beta emissions before it eventually produces a stable 206Pb product 17 Nuclear Stability The nucleons protons and neutrons are held together by the strong nuclear force The strong nuclear force acts through the very short distances between nucleons and overcomes the coulombic repulsion between protons in the nucleus The strong nuclear force acts between protons and neutrons Neutrons help hold the nucleus together Neutrons may dilute protons in the nucleus keeping the protons farther apart to minimize coulombic repulsion These two neutron functions may help explain the need for a larger ratio of neutrons to protons in heavier nuclei 18 Binding Energy A helium 4 atom is composed of 2 protons and 2 neutrons Each proton has a mass of 1 007825 u Each neutron has a mass of 1 008665 u The sum of 2 protons and 2 neutrons is 4 032980 u The experimentally observed mass of helium 4 is 4 002603 u The difference between calculated mass and measured mass is the mass defect m m 0 030377 u for helium 4 The missing mass is converted to binding energy Eb according to Einstein s equation E mc2 Eb 4 5335 10 12 J or 2 7301 109 kJ mol 1 19 Binding Energy The binding energy per nucleon plotted as a function of mass number for elements hydrogen through uranium The curve reaches a maximum at 56Fe 20 Transmutation Fission and Fusion There are three categories of nuclear reactions Transmutation where one nucleus changes into another either by natural decay or in response to some outside intervention Fission a heavy nucleus splits into lighter nuclei Fusion light nuclei merge into a heavier nucleus 21 Transmutation 10B reacts via neutron
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