8. Nuclear Chemistry

1. 8. Nuclear Chemistry Definition: Study of transformations that take place inside the nuclei of atoms. Demos: * fission, fusion http://www.atomicarchive.com/Movies/index.shtml *Decays: http://www.hpwt.de/Kern2e.htm *Fission: http://www.classzone.com/books/earth_science/terc/content/visualizations/es0702/es0702page01.cfm?chapter_no=visualization * Application: Smoke Detector (Am-241)

2. 8.1. Radioactivity Definition: emission of radiations resulting from changes in the nuclei of atoms. Natural Radioactivity: exhibited naturally by some atoms Types of radiations emitted Radiation Symbols Charge Mass (g) Alpha 42He, 42a 2+ 6.65E-24 Beta 0-1e, 0-1b 1- 9.11E-28 Gamma g 0 0 Penetrating ability rank (from most to least penetrating): g(10 cm Pb) , b (0.5 cm Pb), a (paper sheet). RQ2-37

3. RQ2-37 What happens to a nucleus when it produces an alpha particle? a. The atomic number of the nucleus decreases by 2 and its mass number decreases by 4 b. The atomic number of the nucleus decreases by 4 and its mass number decreases by 2 c. The atomic number of the nucleus increases by 2 and its mass number increases by 4

4. 8.2. Nuclear Reactions and Radioactive Decay Nuclear reaction: change of an isotope of one element into another isotope of another element. Rules: *sum of mass #�s of reactants = sum of mass #�s of products * sum of atomic #�s of reactants = sum of atomic numbers of products. Example: 22688Ra -> 42a + 22286Rn a emission causes mass # loss by 4 and atomic # loss by 2

5. Nuclear Reactions (Continued) Example 2: 23992U -> 0-1b + 23993Np b Emission causes atomic # increase by 1 Production of an electron by a nucleus: corresponds to conversion of a neutron to a proton and an electron 10n -> 0-1e + 11p Origin of g radiations: transformation of a nucleus produces an excited nucleus. Return of excited nucleus to ground state emits g radiations

6. Radioactive Decay Definition: process through which a radioactive isotope produces a non-radioactive isotope. Usually takes several steps in which radioactive species are produced. Example: Step 1: 23892U -> 23490Th + 42a Step 2: 23490Th -> 23491Pa + 0-1b Step 3: 23491Pa -> 23492U + 0-1b Step 4: 23492U -> 23990Th + 42a The series continues over 10 more steps. Last product: 20682Pb (non-radioactive)

7. Radioactive Decay Series Complete decay Process: shown as a graph. Example: fig 19.6, pg 874 X axis = atomic number Y axis = mass number Most Graphs Show: * the various transformations that take place. * the half-lives of each isotope: the time it takes for the amount of the isotope to decrease by half.

8. Radioactive Decay Series (Continued 2) Additional examples: * Decay radiations as arrows: http://www.google.com/search?hl=en&source=hp&q=radioactive+decay+series&aq=6&oq=radioactive+decay+&aqi=g10 * no radiations or half-lives: http://www.walter-fendt.de/ph11e/decayseries.htm

9. Radioactive Decay Series (Illustration.) Info Provided * Radioactive decay series of 23592U -> 20782Pb * Particles emitted in 1st 3 steps: a, b, a Info requested * number of a and b particles emitted in the decay * Write equations of 1st 3 steps

10. RQ2-37 What happens to a nucleus when it emits a beta particle? a. Its atomic number decreases by 1 and its mass number stays the same b. Its atomic number stays the sameand its mass number increases by 1 c. Its atomic number increases by 1 and its mass number stays the same

11. Radioactive Decay Series (Exercise: Solution.) Info requested a Find the total mass number loss. Caused by how many a particles? Determine the total atomic # loss caused by the a particles. Difference with actual atomic # loss = ? Caused by how many b particles?

12. Radioactive Decay Series (Exercise: Solution 2.) Info requested b Show the equations of the radioactive decay, indicating the emitted particles in the given order. 235 92U -> 4 2a + ? Step2: b emission Step 3: a emission Extra exercise: #35, pg 898

13. Other types of radioactive decay Positron (0+1b) emission Positron: also known as antielectron Example: 20784Po -> 0+1b + 20783Bi 0+1b emission decreases atomic # by 1 Understand aid: a proton is converted, via the weak force, to a neutron, a positron (also known as the "beta plus particle", and a neutrino (Wikipedia). Electron capture. An extra-nuclear electron is captured by the nucleus. Example: 74Be + 0-1e -> 73Li Electron capture decreases atomic # by 1

14. RQ2-38 Both alpha and positron emissions cause decrease atomic numbers of the splitting nuclei? How are they different? a. Alpha particle decreases the atomic number by 4 while the positron decreases it by 1 b. Alpha particle decreases the atomic number by 1 while the positron decreases it by 2 c. Alpha particle decreases the atomic number by 2 while the positron decreases it by 1

15. Other types of radioactive decay (Exercises) 137N -> 136C +? What particle is emitted and decreases the atomic number? 4120Ca + 0-1e -> ? What event results from the capture of an electron by the nucleus? Extra exercises: #33, pg 898

16. 8.3. Stability of Nuclei http://www.chemcool.com/regents/nuclearchemistry/aim1.htm http://lectureonline.cl.msu.edu/~mmp/kap30/Nuclear/nuc.htm Sallient facts about stable isotopes A. Up to Ca, #protons = #neutrons B. Beyond Ca, #neutrons > #protons C. Beyond Bi, all isotopes are radioactive

17. Predicting the Modes of Radioactive Decay Guidelines for use with isotopes up to Bi. If mass # > atomic wt, isotope has too many neutrons. Result: increase #protons, notably by b-emission. If mass # < atomic wt, isotope has too few neutrons. Result: increase # neutrons, notably by electron capture.

18. Predicting Radioactive Decay (Examples) Predict the probable decay modes for: A. 3214Si What does comparison of 32 and Si Atomic wt suggest about the proportion of neutrons in the nucleus? RQ2-39 What is the subsequent probable decay mode? B. 23994Pu What does comparison of 239 and Pu atomic wt suggest about the proportion of neutrons in the nucleus? What is the subsequent probable decay mode? Extra exercise: #43, pg 899

19. RQ2-39 What does comparison of 32 and Si Atomic wt suggest about the proportion of neutrons in the nucleus and its probable decay mode? a. Si has more neutrons than protons and should undergo electron emission to increase the number of protons b. Si has more protons than neutrons and should undergo electron emission to increase the number of neutrons c. Si has equal numbers of neutrons and protons and should undergo alpha emission to increase the number of protons

20. Rate of Radioactive Decay(Half-Life) Activity: * number of desintegrations per unit time (second). * Proportional to number of radioactive atoms present in a sample. * Measured by Geiger-Muller counter Mathematical relation: DN/Dt = -kN N = activity; DN = change in activity; Dt = change in time; k = decay constant, depends on the isotope

21. Rate of Radioactive Decay(Half-Life, Continued) After integration, DN/Dt = -kN leads to ln(N/N0) = -kt N0 = initial activity of the sample. Half-life: amount of time required for the amount of starting sample to be reduced by half. At half-life, N = � x N0, therefore ln(1/2) = -kt1/2, which leads to t1/2 = 0.693/k Examples: #48 & 50, pg 899 Extra ex: #47 & 49, pg 899

22. Nuclear Binding Energy Definition: energy needed to separate a nucleus into component protons and neutrons Experimental observation: the mass of a nucleus < mass of the sums of component protons and neutrons Mass Defect: Difference between the mass of a nucleus and the mass of its protons and neutrons Binding Energy: related to mass defect.

23. Nuclear Binding Energy Determination Procedure Find the mass defect of an isotope. Use at least six digits after the decimal. Express the resulting mass in kg�s Use the relation Eb = (Dm)c2 to figure out Eb. Resulting units: J/mol of nuclei To find Eb per mole of nucleon, divide Eb/mol of nuclei by the number of nucleons involved. Purpose: gain ability to compare stabilities of various nuclei http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html

24. Eb Determination (Illustration) Example: Find Eb per nucleon of 63Li (MW = 6.015125 g/mol) Dm = (3 x mass of 11H + 3 x Mass of 10n) � mass of 63Li = ((3 x 1.007825 + 3 x 1.008665) � 6.015125) g/mol nuclei Eb = (Dm) x (2.998E8 m/s)2 = Eb per nucleon = Extra exercise: #69, pg 900

25. RQ2-40 If the binding energy decreases starting with Fe, what does that suggest about the trend of the mass defects? A. The mass defects starts decreasing B. The mass defects starts increasing C. No change in mass defect

26. Applications of Nuclear Chemistry http://www.chem.duke.edu/~jds/cruise_chem/nuclear/uses.html Restriction: Healthcare and Chemical Analysis Medical Imaging. A. Technecium-99m. A solution of technetate (9943TcO4(-)) ions are injected into a patient. Images of organs are taken at rest and during strenuous activity. Pictures reveal healthy or abnormal organs. More info: fig 19.17, pg 894 B. Positron Emission Tomography (PET). A positron emitting isotope (ex 15O)is given to the patient. Emitted positrons quickly react with electrons. Result: g particles which are detected.

27. Nuclear Medicine Cancer Treatment. Radiations: used to destroy cancerous cells. Example: 60Co. Drawback: g radiations from 60Co are not selective. Boron Neutron Capture Therapy (BNCT). 10B: not radioactive. Captures neutrons much better than any element. Result:10B + 10n -> 11B -> 7Li + 4a particle. Low penetration of particle limits its work with a few cells.

28. Chemical Analysis Radioactive Isotopes as Tracers Radioactive isotopes are incorporated into regular biological fluids and their travel through the body monitored. Example: 32P in fertilizers, checked in leaves of plants Isotope Dilution A radioactive isotope is injected in bloodstream. After full distribution in the body, the isotope is detected and its concentration measured.

29. RQ2-41 What element desintegrates more slowly? One with a low or one with a high desintegration constant? A. One with a high desintegration constant, because t1/2 = 0.693/k B. One with a low desintegration constant, because t1/2 = 0.693 x k C. One with a low desintegration constant, because t1/2 = 0.693/k