19.1 Nuclear Chemistry

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1 9. Nuclear Chemistry Radio Activity Dr. Fred Omega Garces Chemistry Miramar College RadioActivity

One Winter Day in Chicago The Chain Reaction.

On December 2, Fermi and his colleagues gathered on the balcony of the squash court to test the reactor, slowly withdrawing the last control rod until the "critical," or

Compton telephoned the news to Harvard president James B.

2 One Winter Day in Chicago The Chain Reaction. Construction halted with the fifty-seventh layer on December, when measurements indicated the pile would become selfsustaining should the control rods be withdrawn. On December 2, Fermi and his colleagues gathered on the balcony of the squash court to test the reactor, slowly withdrawing the last control rod until the "critical," or selfsustaining, level was reached, then watching the reactor operate for twenty-eight minutes before reinserting the rod and stopping the reaction. Compton telephoned the news to Harvard president James B. Conant, member of the Manhattan Project Military Policy Committee, with the coded message, "The Italian navigator has just landed in the New World." The first self-sustaining nuclear fission reactor was build on a squash court at the University of Chicago. This accomplishment led to the development of the first atomic bomb at Los Alamos National Laboratory in New Mexico in July 945. This was the dawn of the Nuclear Age. Enrico Fermi was the director. 2 RadioActivity

3 Discovery of Radioactivity 897 Antonio Henri Becquerel Provided the relationship between phosphorescence and X-Rays. Becquerel discovered radiation by accidentally placing a Uranium rock on a photographic plate. He discovered that the rock emitted radiation because of photographic plate exposure. 898 Maria Curie Is credited for naming the strange radiation radioactivity. She went on and won two Noble Prizes for the discovery of Radium and Polonium. Later her daughter also won a Noble Prize. 3 RadioActivity

Radioactivity Nucleons - the subatomic particles in the nucleus: Protons & Neutrons Isotopes - atoms with different number of neutrons in the nucleus but the same number of protons or atomic number

4 Radioactivity Nucleons - the subatomic particles in the nucleus: Protons & Neutrons Isotopes - atoms with different number of neutrons in the nucleus but the same number of protons or atomic number (Z). Isotopes of carbon : C, 2 C, 4 C or C-, C-2, C-3, C-4 (note that Z=6) Uranium Isotopes: U-233, U-235, U-238 (note that Z= 92) The numerical suffixes represent the mass number, A ( Mass number you recall is represented by A; A = protons + neutrons) Radionuclides- Nuclei that are radioactive. Radioisotopes - Atoms containing radionuclides In a Nuclear Equation, the total number of nucleons is conserved 238 U g 234 Th + 4 He Mass number = Atomic number = I g 3 Xe + e Mass number = Atomic number = 54 + (-) 4 RadioActivity

5 Subatomic Particles Nomenclature Particle Charge Mass (g) Nomenclature alpha e He 4 α 2 2 beta - 9.e -28 e β - - gamma γ proton e -24 H p neutron.675 e -24 n electron - 9.e -28 e - positron + 9.e -28 e + 5 RadioActivity

6 Nuclear Equation: Emission Radionuclides - spontaneously emit particles and radiation which can be expressed by a nuclear equation Spontaneous Emission: Mass and charge are conserved. Alpha emission 4 2 He Beta emission - e (product) 3 53 I n Positron Emission p + 5B n e + + (reactant) RadioActivity U 234 9Th He + γ e - e He + Electron capture 234 9Th - e γ 6C p - e + Gamma emission + e 3 54 Xe U p + e - n

7 Radiation Property α β γ Charge +2 - Mass(g) 6.64e e -28 Penetrating Power, Description Low Moderate High Piece paper piece wood Lead wall Velocity 5-7% C > 9% C C Nature of radiation 4 He e γ 2-7 RadioActivity

As the atomic mass increases, the neutron to proton ratio increases because proton-proton repulsion becomes more

8 Patterns of Nuclear Stability Belt of Stability- Neutrons are believed to hold the protons together in the nucleus, like glue. As the atomic mass increases, the neutron to proton ratio increases because proton-proton repulsion becomes more significant. Therefore the neutron to proton ratio must increase for heavier elements. (Note that the belt of stability has a slope greater than indicating an increasing neutron to proton ratio for larger atoms. 8 RadioActivity

9 Belt of Stability: A closer look Neutrons (N) A plot of neutrons vs. protons for the stable nuclides. A plot of N vs. Z for all stable nuclides gives rise to a narrow band that veers above N/Z= shortly beyond Z=. The N/Z values for several stable nuclides are given. The most common modes of decay for unstable nuclides in a particular region are shown: nuclides with a high N/Z ratio often undergo β decay; those with a low ratio undergo e - capture or Positron emission; heavy nuclei beyond the stable band (and a few lighter ones) undergo α decay. The blue box in the larger diagram is expanded to show the stable and many of the unstable nuclides in that area. Note the modes of decay; α decay decreases both N and Z by two; b decay decreases N and increases Z by one; Positron emission and e- capture increase N and decrease Z by one. 9 RadioActivity

10 Pattern to Stability Radioactive decay leads to particles which lie in the Belt of stability Above Belt - β-emitters (High n:p) neutron-rich lowers ratio and move right towards belt of stability. Below Belt - electron capture or positron emitters (Low n:p) proton-rich raise ratio and move left toward belt of stability. Nuclei with Z > 83 tend to be α -emitters Heavy nuclei decrease both proton and neutron. RadioActivity

11 Other Considerations Other Factors to Nuclear Stability Magic number Protons with - 2, 8, 2, 28, 5 or 82 Neutrons with - 2, 8, 2, 28, 5, 82 or 26 Nuclei with even # of protons and neutrons are more stable than with any odd number of protons and neutrons. Shell model of the nucleus explains these observation. Magic number correspond to filled, closed-shell nucleon configuration. Pairs of protons and neutrons analogous to pair of electrons in the atom. RadioActivity

Nuclear disintegration series for U-238 under goes α- emission (blue arrows) and β-

12 Predicting Nuclear Stability - Radionuclides sometimes go through a series of emission (Radioactive series) before becoming a stable nuclei. Nuclear disintegration series for U-238 under goes α- emission (blue arrows) and β- emission (red arrows) until it forms stable Pb-26 (which is an isotope within the belt of stability. Radioactive Series 2 RadioActivity

13 Transmutation - Change of nuclear identity by artificially striking nucleus with a particle. Many medical chemotherapy isotopes are formed by transmutation. Many new elements are discovered by transmutation Nomenclature: Target (bombard, ejected) product 8 7 O + H g 24 He N In this example, The target is 7 O, the product is 4 N, the bombarding particle is a proton H (or p) and the ejected particle is the alpha particle 4He (or α). The nomenclature is therefore 7 O (p,α) 4 N Nuclear Transmutation Example: solve the following. The answer is in the next slide. i) 238 U + n g 239 Np + β ( see next slide for answer) ii) 238 U (n,γ) 239 U ( see next slide for answer) iii) 8 O (n,β) 9 F ( see next slide for answer) 3 RadioActivity

Cyclotrons or synchrotron Chemotherapy Application Answer for previous

14 Transmutation: Charge and neutral particles Particle Accelerators - Slamming particles into nuclei leads to synthesis of different or new elements Cyclotrons or synchrotron Chemotherapy Application Answer for previous problems: (i) 238 U(n, β) 239 Np (ii) 238 U + n g γ U (iiii) 8 O + n g β + 9 F 4 RadioActivity

15 Transmutation: Transuranium Artificial transmutation used to produce elements above 92. Neutron projectiles are bombardment source. These come from radioactive isotopes. 238 U + n g 239 U g 239 Np + - e 239 Np g 239 Pu + - e Example 2.55 a) 235 U + n g 6 Sm + 72 Zn +? n b) 239 Pu + n g 44 Ce +? + 2 n Example 2.56 b) 233 U + n g 33 Sb + 98 Nb +? n 5 RadioActivity

16 Rates of Radioactive Decay Radioactive substance possess special rates of decay and half lives, t /2. Radioactive decay is a first-order kinetic process. ln {N/N o } = -k t t /2 =.693/ k Half-life - The time in which a substance will decay to onehalf its original mass. 6 RadioActivity

17 Radioisotope Half-Lives Each isotope has its characteristic half-life unaffected by external conditions, i.e., temp, pressure, chemical nature. Half-Lives and Type of Decay for Several Radioisotopes. Isotope Half-life (yr) Type of Decay Natural radioisotopes 238 U Alpha 235 U 7. 8 Alpha 232 Th.4 Alpha 9 4 K.3 9 Beta 6 4 C 5,73 Beta Synthetic radioisotopes Pu 24, Alpha Cs 3 Beta 38 9 Sr 28.8 Beta 53 3 I.22 Beta 7 RadioActivity

18 Radioisotope Dating For st Order Reaction: Half-life is independent of concentration of reactant. Otzi The Iceman ln[a] = - kt + ln[a] o ln [A] o "[A] o % $ # 2 ' & t 2 = ln2 k = k t /2 C-4 dating is accurate only up to 5,yr. 4 C g 4 N + - e β-emission U-238 accurate up to yr. Based on data, Earth is Billion yrs old. 8 RadioActivity Otzi the iceman, discovered Sept 9, Carbon dating ~ 53 yrs old

Calculation of Age Based on t /2 TURIN, Italy -- Almost everything about the Shroud of Turin is mysterious- its age, its authenticity, and the identity of the bearded man with deep-set eyes whose

19 Calculation of Age Based on t /2 TURIN, Italy -- Almost everything about the Shroud of Turin is mysterious- its age, its authenticity, and the identity of the bearded man with deep-set eyes whose image is imprinted on the 4-foot length of yellowing linen, still believed by many Christians to be the burial cloth of Jesus....as carbon testing done on tiny swatches of the shroud concluded in or to the time of Jesus, the centuries-old fascination with the shroud... t 2 t 2 = ln2 k = 573 yr k = ln2 t 2 = 2 C 4 C = 2 [ 4 C]= 5. ppt based on 2 C [ 4 C]= 46.4 ppt today ln2 573 yr = yr =.2-4 yr The Shroud of Turin is a linen cloth over 4 m long. It bears a faint, strawcolored image of an adult male of average build who had apparently been crucified. Reliable records of the shroud date to about 35, but for these past 6 years it has been alleged to be the burial shroud of Jesus Christ. Numerous chemical and other tests have been done on tiny fragments of the shroud in recent years. The general conclusion has been that the image was not painted on the cloth by any traditional method, but no one could say exactly how the image had been created. Re-cent advances in radiochemical dating methods, however, led to a new effort in to estimate the age of the cloth. Using radioactive 4 C, the flax from which the linen was made was shown to have been grown between 26 and 39 A.D. There is no chance that the cloth was made at the time of Christ. A o = 5. " Ao # =ln A = 46.4 $ A = kt 5. ln yr = t t = yr = 32 ±5 AD 9 RadioActivity

20 Rates of Radioactive Decay Example: The half life for 238 U g 26 Pb is yr. A mineral sample contains 5. mg of 238 U and 4. mg of 26 Pb. What is the age of the mineral? Original U = 5 mg U + 4. mg Pb 238 U = 66.2 mg U 26 Pb For First order Kinetics: ln[a] = - kt + ln[a o ] t /2 = ln2 k =.693 =.5 - yr - k yr ln[a] = - kt + ln[a o ] kt = ln [A o ] - ln[a] kt = ln [A o ] [A] t = ln [A o ] k [A] t = ln [A o ] = ln 66.2 k [A t ].5 - yr - 5. t =,87,49,77 yrs t =.9 9 years or.9 billion years 2 RadioActivity

21 Example: Rates of Radioactive Decay The half-life of U-238 is yr. A sample of rock of mass.7 g is found to produce 3 dis/s. Calculate the % mass 238 U. Rate = k [N], % 238 U = N N o, N o =.7 g k =.693 t /2 = yr =.54 - yr - 3 dis s 6 s min 6 min hr 24 hr day day atom yr dis mole atom 238 g U mole = g -3 yr Rate = K[N] [N] = Rate K = g yr = g.54 yr % mass 238 U = g =.48 %.7g 2 RadioActivity

22 Summary Radioactivity Process Type of Radiation α, β, γ, positron Nuclear Equation Nucleons are conserved Nuclear Stability Greater Z, the higher n:p ratio Transmutation Changing atomic identity Rates of Decay Radioactivity follows first order Kinetics 22 RadioActivity

Chapter 21. Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten

Chapter 21. Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten , The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten Chapter 21 John D. Bookstaver St. Charles Community College St. Peters, MO 2006, Prentice Hall, Inc. The