Chapter 20: Nuclear Chemistry Essay

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Nuclear chemistry
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chemistry of the nucleus rather than the electrons.
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Chemical Reaction
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∙ does NOT change the nucleus ∙ different ISOTOPES behave essentially the SAME ∙rate is affected by temp, pressure, or catalyts ∙atoms in compounds behave differently than elements ∙relatively small energy change compared to nuclear reactions
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Nuclear Reaction
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∙ CHANGES the nucleus ∙ different ISOTOPES often behave DIFFERENTLY ∙rate is UNAFFECTED by temp, pressure (within the range found on Earth), or catalyst ∙atom behaves the SAME whether it is elemental or in a compound ∙energy change can be millions of times greater than a chemical reaction
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nucleons
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the nucleus is comprised of the two of these: protons and neutrons
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atomic number
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number of protons, determines element
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mass number
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number of protons and neutrons added together
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isotopes
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∙NOT all atoms of the same element have the same mass. ∙They can have DIFFERENT numbers of NEUTRONS, and thus different MASS numbers. ∙Atoms that have the same atomic number but different mass numbers are isotopes of each other
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radioactivity
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Spontaneous emission of particles or electromagnetic radiation from the nucleus The process is called *radioactive decay* or *nuclear decay.*
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nuclear transmutation
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bombardment of nuclei by neutrons, protons, or other nuclei.
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Nuclei decay into different nucleus:
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∙Alpha (a) decay: –>the nucleus loses TWO protons and TWO neutrons (i.e. helium nucleus) ∙Beta (b) decay: –>neutron→ proton + ejected electron ∙Gamma (g) emission: –>NO change in mass or atomic number ∙Positron emission (PE): –>proton→ neutron + ejected positron ∙electron capture (EC) –> proton + inner shell electron → neutron
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The symbols for subatomic particles:
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∙ proton ¹₁H ¹₁p ∙neutron ¹₀n ∙electron ⁰₋₁e ⁰₋₁B ∙positron ⁰⁺₁e ⁰⁺₁B ∙alpha particle ⁴₂a ⁴₂He
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nuclear reactions
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A nuclear equation is balanced when the number of nucleons and the sums of the charges are the same on both sides protons + neutrons→ protons→ 92 protons+ 146 neutrons 90 protons+ 144 neutrons 2 protons+ 2 neutrons
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Nuclear Stability
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**neutron-to-proton ratio (n/p) –> There are more stable nuclei with 2, 8, 20, 50, 82, or 126 protons or neutrons –>More with even #’s –>All with atomic number > 83 are radioactive –>All isotopes of ₄₃Tc and ₆₁Pm are radioactive
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belt of stability
belt of stability
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∙Stable nuclei are located in this area of the graph ∙Most radioactive nuclei lie outside the belt. ∙Above the belt of stability, the nuclei have higher neutron-to-proton ratio.
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Alpha Decay
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∙the loss of an a-particle (a helium nucleus). ⁴₂He or ⁴₂a *The nucleus loses two protons and two neutrons
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Beta decay
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∙the loss of a B-particle (a high energy electron). no nucleons ↓ ⁰₋₁B or ⁰₋₁e
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Beta emission
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*decomposition of a neutron to yield an ELECTRON and a PROTON. –>Because it is from decomposition of a **neutron**, the ELECTRON is coming from the *nucleus*, NOT from the orbitals. –>It is ejected from the nucleus. –>atomic number INCREASES by 1 ₀¹n → ¹₁p + ⁰₋₁e (n→ p +) *e⁻*)→→→
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Gamma radiation
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∙the loss of a y-ray, which is made up of high-energy photons (electromagnetic radiation of very short wavelength). ∙Almost always accompanies the LOSS of a NUCLEAR particle, but is usually omitted from nuclear equations. ∙Emission of gamma rays causes NO change in mass or atomic number
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Positron Emission
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∙involves the conversion of a proton in the nucleus into a neutron plus an ejected positron. proton → neutron + positron ∙atomic number DECREASES by 1 ¹₁p → ¹₀n + ⁰₁e (p→ n + (*E+*)→→
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positron
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∙particle that has the same mass as but an opposite charge to that of an electron. ∙are very short-lived. When they collide with electrons, they are converted to gamma rays. positron + electron → 2 gamma rays →no nucleons⁰₁B or ⁰₁e
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electron capture
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∙electron is captured from the inner shell of the surrounding electron cloud. (¹₁p) proton + (⁰₋₁) electron → (¹₀n) neutron *atomic number DECREASES by 1
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Nuclear Stability
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∙Nuclei ABOVE the “belt of stability” have too many NEUTRONS. ∙They tend to decay by emitting *BETA particles.* ¹₀n → ¹₁p + ⁰₋₁e _______________________________ ∙Nuclei BELOW the belt have too many protons ∙They tend to become MORE STABLE by *positron emission* or *electron capture.* ¹₁p → ¹₀n + ⁰₁e or ¹₁p + ⁰₋₁e → ¹₀n ∙There are NO stable nuclei with an atomic number greater than 83 (no more “belt of stability”). i.e. All nuclei with more than 83 protons are radioactive. ∙Nuclei with such large atomic numbers (>83) tend to decay by *alpha emission.* ⁴₂He or ⁴₂a₂ ∙ Large nuclei emit large particles.
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radioactive decay series
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is a sequence of nuclear reactions that ultimately result in the formation of a stable isotope.
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parent isotope
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the beginning radioactive isotope
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daughter isotope
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the product isotope
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Einstein’s famous equation expresses this:
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E = mc² where E = energy, m = mass, and c = the speed of light (2.9979 x 10⁸ m/s). **The law of conservation of energy = The law of conservation of mass
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nuclear binding energy
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is the energy required to break up a nucleus into its component *protons and neutrons.*
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nucleons (protons and neutrons)
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The mass of a nucleus is always LESS than the masses of the individual nucleons (protons and neutrons) it is composed of. =this is called=> *mass defect*
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Loss in mass is converted to energy and can be quantified with Einstein’s mass-energy equivalence relationship:
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ΔE = (Δm)c² →ΔE = energy of product – energy of reactant →Δm = mass of product – mass of reactant ∙(= mass of nucleus – sum of mass of nucleons) ∙(= mass lost when nucleus is formed)
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Nuclear Binding Energy
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= -2.3686 x 10-11 J = energy lost (stability gained) when the nucleus of one atom is formed *the energy released when nucleons combine to form a nucleus; formation of the nucleus is energetically favorable *nuclear binding energy: the energy required to break up a nucleus into its component protons & neutrons
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Binding Energy per Nucleon
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(Nuclear Binding Energy) / (number of nucleons) **highest BEPN is the most stable (requires the most energy per nucleon to breakup the nucleus) **nuclei w/ mass number ~40-100 have the highest Binding Energy per Nucleon, and thus are the most stable.
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Kinetics (Rates) of Radioactive Decay
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∙Radioactive decay is a first-order process. ∙ The rate depends only on the number of radioactive nuclei N in the sample. Rate = kN where k is now called the *decay constant*,N is the number of *radioactive nuclei* in the sample
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activity
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∙rate at which the sample decays –>expressed in units of *bequerels (Bq)*, which equals 1 nuclear disintegration per second. –>*curie (Ci) * is 3.7 x 1010 disintegrations/second.
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Integrated rate law:
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ln Nt/N₀ = -kt where t= time interval of decay N₀= initial number of nuclei (at t=0) Nt= remaining number of nuclei after time t. –>Since mass and activity are each proportional to N, a *mass/mass ratio* or an *activity/activity ratio* can be used instead of N/N.
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half-life, t½
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is the time for half of the material to decay. So for t½ , Nt / N₀ = ½ and thus: ** 0.693/k = t₁/₂
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radiometric dating
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Because the half-life of a particular nuclide is constant, by: *comparing the % abundance of a radioactive isotope at a given point in time with the % abundance normally present,* one can find the age of an object. ∙Living systems take up ¹⁴C, but no longer do when they die. ∙By measuring the amount of ¹⁴C present compared to the natural abundance of ¹⁴C, an approximate age of a fossil can be determined.
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Nuclear transmutations
Nuclear transmutations
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nuclear reactions induced by colliding a neutron or another nucleus with the nuclide. ²⁷₁₃Al + ¹₀n→ ²⁴₁₁Na + ⁴₂He
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Shorthand way of summarizing a nuclear transmutation:
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∙target nucleus (bombarding particle, ejected particle) product nucleus ²⁷₁₃Al (n,a) ²⁴₁₁Na ∙n is used for neutron, ∙p is used for proton, ∙a is used for alpha particle
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Particle accelerators
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Charged particles must be moving at high speed to have enough energy to *overcome charge-charge repulsion* and *collide with the nucleus.* –> A particle accelerator is used.
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neutron capture
neutron capture
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Combination of a nucleus with a neutron takes place through this.
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transuranium elements
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∙Elements above atomic number 92 (Uranium) ∙ because they follow Uranium on the periodic table. All of them are radioactive ∙can be made by bombardment of smaller nuclei with neutrons or other nuclei. e.g. bombardment with an accelerated a-particle (a charged particle of 2 protons and 2 neutrons):
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Nuclear Fission and Fusion
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∙When small nuclei are *fused* into a larger nuclei of the same number of nucleons, *energy is released.* ∙ when large nuclei are fragmented *(fissioned)* into mid-sized nuclei with the same total number of nucleons, *energy is released.*
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Nuclear fission
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∙a heavy nucleus (mass number > 200) divides to form smaller nuclei and one or more neutrons. ∙Heavy nuclei gain stability (release energy) when fragmented into mid-sized nuclei. –> Bombardment of the radioactive nuclide with a neutron starts the process.
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nuclear chain reaction
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self-sustaining sequence of nuclear fission →Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.
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critical mass
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minimum amount of fissionable material present for the chain reaction to be sustained
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Nuclear Reactors
Nuclear Reactors
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In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator
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control rods
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capture some neutrons, keeping the system from reaching a dangerous supercritical mass
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Light reactors
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use water as a coolant and uranium-235 as fuel, usually in the form of U₃O₈. The U-235 must be enriched to 3-4%.
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Heavy reactors
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use D₂O, and the U-235 does not need to be enriched.
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Breeder reactors
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add U-238, plutonium-239, or thorium-232 to the U-235. Each of these produces more fissionable material as it absorbs neutrons.
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Nuclear Fusion
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Light nuclei are combined (fused) together to give heavier nuclei. –>Thermonuclear reactions: must be in the plasma state at several million kelvins
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Hydrogen bomb
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Requires the energy from a fission detonation to initiate the following fusion reactions: ⁶₃Li +²₁H→ 2⁴₂a ²₁H +²₁H→ ³₁H + ¹₁H
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tracers
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Use of tracers for diagnosis include: ∙Sodium-24: blood flow ∙Iodine-131: thyroid activity ∙Iodine-123: brain imaging ∙Technetium-99: organ imaging
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Ionizing Radiation
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is radiation that removes an electron from an atom or molecule, making an ion. –> alpha, beta, gamma
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Geiger counter
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radiation creates ions, which conduct a current
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hydroxyl radical
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–>Ionization radiation in living tissue removes electrons from water. ∙OH, a neutral species with an unpaired electron. Radicals react with many things in a manner that generates more free radicals, so the resulting chain reaction can do a lot of damage in a biological system.
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the rad (radiation absorbed dose).
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common unit for the absorbed dose of radiation 1 rad = 1 x 105 J/g of tissue irradiated
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rem (roentgen equivalent for man)
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is determined from the number of rads: Number of rems = (number of rads) x (RBE) ∙RBE = the relative biological effectiveness. ∙A dose of 600 REMs will kill most humans

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