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U13 – Nuclear Chemistry Chapter 19 19.1 Nuclear Stability and Radioactive Decay Review • Atomic Number (Z) – number of protons • Mass Number (A) – sum of protons and neutrons A Z X 19.1 Nuclear Stability and Radioactive Decay • Nucleus undergoes decomposition to form a different nucleus. • Nuclides with 84 or more protons are unstable. • Light nuclides are stable when Z equals A – Z (neutron/proton ratio is 1). • For heavier elements the neutron/proton ratio required for stability is greater than 1 and increases with Z. 19.1 Nuclear Stability and Radioactive Decay • Certain combinations of protons and neutrons seem to confer special stability. Even numbers of protons and neutrons are more often stable than those with odd numbers. • Certain specific numbers of protons or neutrons produce especially stable nuclides. 2, 8, 20, 28, 50, 82, and 126 19.1 Nuclear Stability and Radioactive Decay Zone of Stability 19.1 Nuclear Stability and Radioactive Decay 19.1 Nuclear Stability and Radioactive Decay Types of Radioactive Decay • Alpha production ( ): • Beta production ( ): A neutron is converted into a proton and electron 19.1 Nuclear Stability and Radioactive Decay Types of Radioactive Decay • Gamma ray production ( ) w/ alpha particle emission: Gamma rays are high energy photons • Positron production: A proton is converted to a neutron and a positron (an electron w/ a positive charge) 19.1 Nuclear Stability and Radioactive Decay Types of Radioactive Decay • Electron capture: Inner-orbital electron The nucleus absorbs an electron changing a proton to a neutron 19.2 Kinetics of Nuclear Decay Rate of Decay Rate = kN • The rate of decay is proportional to the number of nuclides. This represents a first-order process. Half Life • Time required for the number of nuclides to reach half the original value • Amount = original amount x (.5)(# half lives) • Or ln 2 0.693 t1/ 2 = = k k 19.3 Nuclear Transformations The sum of the A values on either side of the arrow must be equal. The same is true for the Z values. 27 13 249 98 Al + He 4 2 30 15 1 0 P+ n 263 Cf + 188 O 106 Sg + 4 01 n Figure out the type of radiation first, then determine A and Z values, and then identify elements. 19.4 Detection and Uses • This section is not directly tested on the AP exam. • They may ask how radiation is used in industry, but that is all. Please read it. – Medical uses – Radioactive dating – Geiger Counters 19.5 Thermodynamic Stability of the Nucleus Energy and Mass • When a system gains or loses energy it also gains or loses a quantity of mass. E = mc2 m = mass defect E = change in energy • If E is negative (exothermic), mass is lost from the system. 19.5 Thermodynamic Stability of the Nucleus Mass Defect (Δm) • Calculating the mass defect for 42 He: Since atomic masses include the masses of the electrons, we must account for the electron mass. 4.0026 = mass of 1.0078 = mass of • 4 2 He 1 1H atom = mass of atom = mass of 4 2 He 1 1H nucleus + 2me nucleus + me 4 2 He nucleus is “synthesized” from 2 protons and two neutrons. m = 4.0026 2me m = 0.0304 amu 2 1.0078 me + 2 1.0087 19.5 Thermodynamic Stability of the Nucleus • The energy required to decompose the nucleus into its components. • Iron-56 is the most stable nucleus and has a binding energy of 8.97 MeV. 1 eV (electron-volt) is a unit of energy equal to 1.602 x 10-19 J. It is related to the energy of an electron as it accelerates through a potential difference of one volt. So it would take (8.97 x 109) x 1.602 x 10-19 J to break apart one iron atom. It would take (6.0221 x 1023) x (8.97 x 109) x (1.602 x 10-19) to break apart 56g (1 mole) of iron. That would be 8.65 x 1013 J. This is roughly equivalent to 1.15 x 107 kg or 12,600 tons of dynamite. 19.5 Thermodynamic Stability of the Nucleus 19.6 Nuclear Fission and Nuclear Fusion • Fusion – Combining two light nuclei to form a heavier, more stable nucleus. • Fission – Splitting a heavy nucleus into two nuclei with smaller mass numbers. 1 0 n+ 235 92 U 142 56 Ba + 91 36 1 0 Kr + 3 n 19.6 Nuclear Fission and Nuclear Fusion • A self-sustaining fission process is called a chain reaction. Neutrons Causing Fission Event Event subcritical <1 critical =1 supercritical >1 Result reaction stops sustained reaction violent explosion 19.6 Nuclear Fission and Nuclear Fusion Nuclear Power Plant Schematic 19.6 Nuclear Fission and Nuclear Fusion Nuclear Core Schematic 19.7 Effects of Radiation Depend on: 1. Energy of the radiation 2. Penetrating ability of the radiation 3. Ionizing ability of the radiation 4. Chemical properties of the radiation source 19.7 Effects of Radiation • The energy dose of the radiation and its effectiveness in causing biologic damage must be taken into account. Number of rems = (number of rads) × RBE rads = RBE = radiation absorbed dose relative effectiveness of the radiation in causing biologic damage