John W. Moore Conrad L. Stanitsi Peter C. Jurs http://academic.cengage.com/chemistry/moore Chapter 9 Nuclear Chemistry Stephen C. Foster Mississippi State University The Nature of Radioactivity Henri Becquerel (896): U salts emitted rays that fog a photographic plate. U metal was a stronger emitter. Marie and Pierre Curie: Isolated Po and Ra that did the same. Marie Curie called the phenomenon radioactivity. Thomson and Rutherford: Studied the radiation, and found two types: α and β. Villard: Discovered g radiation. The Nature of Radioactivity Name Symbol Charge Mass (g) Pen. Power* alpha He + 7 x -. mm beta b e 9 x -8 mm gamma g mm *Distance at which half the radiation has been stopped by water. Nuclear Reactions Rutherford & Soddy (9) Radioactivity is the result of a natural change of a radioactive isotope of one element into an isotope of a different element. 6 Ra Rn + He 88 86 Radium-6 Radon- alpha particle mass no. (A) 6 + atomic no. (Z) 88 86 + Note: A and Z must balance. Alpha and Beta Particle Emission Alpha a nucleus ejects a helium nucleus: 8 9 9 U Th + He Beta a nucleus ejects an electron: 9 8 Sr 9 Y + 9 e How does a nucleus eject an e -? A series of steps, but the net result is: n p + e neutron proton electron Radioactive Series A decay product (daughter isotope) is often unstable... A radioactive series. The neutron number: N = A - Z
Other Types of Radioactive Decay Positron emission Positron = positive electron ( e or + ). Antimatter. + + Sc Ca + e Other Types of Radioactive Decay Electron capture (EC) An inner-shell e - (K shell) is captured by the nucleus. 7 Be + e 7 Li Sometimes called K-capture. Antimatter is annihilated by collision with matter: + + e - Nuclear Equations Radioactive iodine is used to test thyroid function. It undergoes beta decay to form a new element. Write a balanced equation for the process. Stability of Atomic Nuclei Loo up Z for I (Z = 5): 5 I Add (product): Calculate the Z and A for the new isotope: 5 I e + 5 I e + 5 Xe Element 5 The Band of Stability Stable nuclei have N Z. Nuclei with Z < : N / Z. Nuclei with Z > : N / Z gradually increases. 9 Bi (Z = 8) is the heaviest stable nucleus. Even-Z isotopes are more common than odd. Even-N isotopes are more common than odd. even-even ; odd-even ; odd-odd Unstable isotopes decay so that the daughter will enter the peninsula of stability. Band of Stability & Type of Decay Elements with Z > 8 Most decay by alpha emission. Elements with Z<8 Use a periodic table Compare A with the element s average atomic wt. Too heavy (excess n ): emission (n p + + e - ). Too light: + emission or e - capture (p + n ).
Band of Stability & Type of Decay Example Predict how 8 P will decay. Atomic weight of P =.97 8 P is too light. β + decay. 8 P e + 5 Example How will 8 Mg decay? 8 Mg is too heavy. β decay. Na.99 Mg. Al 6.98 Si 8.9 5 P.97 8 Mg e + + 6 S.7 8 Al 8 Si Binding Energy A measure of the force holding a nucleus together. E b = ΔE nucleus formation E released when component p + + n combine. Einstein (special relativity): E = mc with: E b = -ΔE = -(Δm) c Δm = (mass nucleus) (mass of p + + n ) c = speed of light =.997958 x 8 ms =. x 8 ms Nuclear Binding Energy Determine the binding energy and binding energy per nucleon for C. The mass of C =. g/mol, m n =.867 g/mol, and m p =.78 g/mol. Nuclear Binding Energy Determine the binding energy and binding energy per nucleon for C. Δm =.99 g/mol = 9.9 x -5 g/mol 6 n : 6 x.867 = 6.5 6 p + : 6 x.78 = 6.698 Total mass nucleons =.99 g/mol Δm = mass of nucleus sum of nucleons =..99 g/mol = -.99 g/mol ΔE = 9.9 x -5 g/mol (.998 x 8 m/s) ΔE = 8.898 x g m s - mol E b = ΔE = +8.9 x J mol (J = g m s - ) Since C has nucleons: E b /nucleon = (8.9 x / ) = 7. x J mol Nuclear Binding Energy E b /nucleon for stable isotopes: Rates of Disintegration Reactions Radioactive decay is st order: ln [X] t = t + ln [X] [X] = initial concentration of isotope X [X] t = concentration of X after time t = rate constant. Half life: t ½ = ln =.69
Half Life t / ( 9 Pu) =, years: Half Life Decay Process Half-life 8 Th + He 9 9.5 x 9 y H He + e. yr 6 5 5 57 8 5 9 8 6 7 C N + e 57 yr 7 I Xe + e 8. d 5 I + e Te. h 5 57 Cr Mn + e s 5 8 P Si + e.7 s + 9 Sr Y + e 8.8 yr 9 6 Co Ni + e 5.6 yr 8 Half-Life 9 Ir decays with a rate constant of 9. x - d (a) What is t / for 9 Ir? (b) What fraction of a 9 Ir sample would remain after days? (a) t / = (ln )/ = (.69)/(9. x - d ) = 7.5 d N (b) ln = -t = -(9. x N - d )( d) = -.9 Rate of Radioactive Decay The activity (A) of a sample of N atoms: A = (disintegrations/time) observed. A = (constant) N constant = if all decays are detected At t = the activity A = (constant) N At a later time, t A = (constant) N N N = e -.9 =.9 9% of the original sample remains. Then: A = N = fraction of atoms remaining A N Rate of Radioactive Decay Since: Rate of Radioactive Decay Geiger counter: an Ar-filled tube under high voltage. ln N t = t + ln N or ln N = -t N ln A A = -t ln As usual t ½ = =.69 becquerel (Bq) = disintegration/sec (s ). curie (Ci) =.7 x s = decay rate of g of Ra.
Carbon Dating High-energy cosmic rays eject n from atoms in the upper atmosphere. C is produced by collision: 7 6 N + n C + H World-wide production of C 7.5 g/year. It is: Evenly distributed Converted into CO, then sugars (photosynthesis). Mammals eat the plants Activity (living organisms) = 5. min g of carbon. Carbon Dating After death the uptae stops. Stored C decays. t ½ ( C ) = 5.7 x years. Used to measure up to 9 half-lives ( 5, years) A = 5. min g A 5,y =. min g h g Longer times are difficult to measure reliably. Prehistoric cave painting Nuclear Fission Hahn and Strassman (98) fired n at 5 U. Ba was produced! Nuclear fission had occurred. 5 9 U + n 6 9 U 56 Ba + 9 Kr + 6 n n produced Very exothermic Nuclear Fission Chain reactions are possible: Small amounts of 5 U can t capture all n. (stays under control). Nuclear bombs exceed the critical mass; the chain reaction grows explosively. Nuclear Reactors Natural U is 99.% 8 U (not fissile). E fission ( 5 U) = x J/mol. g of 5 U ilotons of TNT. Reactor fuel rods are enriched to % 5 U. Weapons-grade is > 9% 5 U. Nuclear Reactors Nuclear power-plants produce clean energy. No atmospheric pollution. No CO. But yield highly radioactive waste Tens of thousands of tons in storage Long half-lives ( 9 Pu, t / =, yr) Can be vitrified (encased in glass ) V waste = m /reactor/yr. Yucca Mountain, NV (salt dome). nuclear plants in the U.S. None built since 979 (Three Mile Island). 5
Nuclear Fusion Light atoms can be joined: H He + e Nuclear fusion. Very exothermic (ΔE = -.5 x 9 J/mol ). The energy source for stars. An attractive power source: Hydrogen (the fuel) can be extracted from oceans. Waste products are short-lived, low-mass isotopes. + Fusion powers the sun Nuclear Fusion Unfortunately, fusion is hard to produce on earth: H-atoms must be converted into a plasma a soup of bare nuclei and e -. T > 8 K required. The plasma is hard to contain, magnetic bottles are used. Commercial fusion reactors are not very liely to occur in the near future. Nuclear Radiation: Effects & Units rad radiation absorbed dose. rad =. J absorbed/g of material Nuclear Radiation: Effects & Units,, and have different biological effects, so rem roentgen equivalent in man. gray (Gy) SI unit. Gy = J absorbed/g of material Gy = rad dose in rem = (quality factor) x (dose in rads) sievert (Sv) SI version. Sv = rem Roentgen (R) dosage of X-ray and -radiation. R = 9. µj deposited/g of tissue Quality factors: = -, =, = Bacground Radiation Radon Produced by naturally occurring U-deposits in the soil. An inhalation hazard: 86 8 8 Rn Po + He Po(s) remains in the lungs and decays: 8 8 8 Po Pb + He A common household hazard. Key: Source % of total (millirems/yr) 6