Chapter 4 Nuclear Chemistry: The Heart of Matter

1. Chapter 4 Nuclear Chemistry: The Heart of Matter

2. Radioisotopes • Radioactive decay – Many isotopes are unstable • Nuclei that undergo radioactive decay • May produce one or more types of radiation

3. Natural Radioactivity • Background radiation • What occurs from natural sources • Accounts for >80% of radioactivity exposure

4. Types of Radiation • Ionizing radiation – knocks electrons out of atoms or groups of atoms • Produces charged species – ions • Charged species that cause damage

5. Alpha Decay • Nucleus loses  particle • Mass decreases by 4 and atomic number decreases by 2

6. Beta Decay • Nucleus loses  particle • No change in mass but atomic number increases Positron Emission • Loses a positron • Equal mass but opposite charge of an electron • Decrease in atomic number and no change in mass • +

7. Electron Capture • Nucleus absorbs an electron and then releases an X-ray • Mass number stays the same and atomic number decreases Gamma Radiation • Release of high-energy photon •  • Typically occurs after another radioactive decay • No change in mass number or atomic number

8. Nuclear Equations • Elements may change in nuclear reactions • Total mass and sum of atomic numbers must be the same • MUST specify isotope

9. Differences Between Chemical and Nuclear Reactions

10. Example4.1  Balancing Nuclear Equations Solution a. We start by writing the symbol for plutonium-239 and a partial equation showing that one of the products is an alpha particle (helium nucleus): 239 94 239 94 235 92 4 2 4 2 U Pu Pu He + ? He + Mass and charge are conserved. The new element must have a mass of 239 – 4 = 235 and a charge of 94 – 2 = 92. The nuclear charge (atomic number) of 92 identifies the element as uranium (U): Write balanced nuclear equations for each of the following processes. In each case, indicate what new element is formed. a. Plutonium-239 emits an alpha particle when it decays. b. Protactinium-234 undergoes beta decay. c. Carbon-11 emits a positron when it decays. d. Carbon-11 undergoes electron capture.

11. Half-Life • Period for one-half of the original elements to undergo radioactive decay • Characteristic for each isotope • Fraction remaining = n = number of half-lives

12. Solution The fraction remaining after three half-lives is 1 1 1 1 = = = 2n 23 8 2 x 2 x 2 1 The amount of cobalt-60 remaining is ( ) (400 mg) = 50 mg. 8 Exercise 4.2A You have 1.224 mg of freshly prepared gold-189, half-life 30 min. How much of the gold-189 sample remains after five half-lives? Exercise 4.2B What percentage of the original radioactivity remains after five half-lives? Example4.2  Half-Lives You obtain a new sample of cobalt-60, half-life 5.25 years, with a mass of 400 mg. How much cobalt-60 remains after 15.75 years (three half-lives)?

13. Solution There are 120 min in 2 hr. There are ( ) = 6 half-lives in 2 hr. The fraction remaining after six half-lives is 120 20 1 1 1 1 2n 26 = = = 64 2 x 2 x 2 x 2 x 2 x 2 The amount of mercury-190 remaining is ( ) (20.0 mg) = 0.313 mg. 1 64 Exercise 4.3A A sample of 16.0 mg of nickel-57, half-life 36.0 hr, is produced in a nuclear reactor. How much of the nickel-57 sample remains after 7.5 days? Exercise 4.3B Tc-99 decays to Ru-99 with a half-life of 210,000 years. Starting with 1.0 mg of Tc-99, how long will it take for 0.75 mg of Ru-99 to form? Example4.3 You obtain a 20.0-mg sample of mercury-190, half-life 20 min. How much of the mercury-190 sample remains after 2 hr?

14. Radioisotopic Dating • Use certain isotopes to estimate the age of various items • 235U half-life = 4.5 billion years • Determine age of rock • 3H half-life = 12.3 years • Used to date aged wines Carbon-14 Dating • 98.9% 12C • Produce 14C in upper atmosphere • Half-life of 5730 years • ~50,000 y maximum age for dating

15. Solution The carbon-14 has gone through three half-lives: 1 ( ) 1 1 1 1 3 8 = = x x 2 2 2 2 It is therefore about 3 x 5730 = 17,190 years old. Exercise 4.4 How old is a piece of cloth that has carbon-14 activity that of new cloth fibers? The half-life of carbon-14 is 5730 years. Example4.4   A piece of fossilized wood has carbon-14 activity one-eighth that of new wood. How old is the artifact? The half-life of carbon-14 is 5730 years. 1 16

16. Shroud of Turin • Alleged burial shroud of Jesus Christ • Contains faint human likeness • First documented in Middle Ages • Carbon-14 dating done in 1988 • Three separate labs • Shroud ~800 years old • Unlikely to be burial shroud Artificial Transmutation • Transmutation changes one element into another • Middle Ages: change lead to gold • In 1919 Rutherford established protons as fundamental particles • Basic building blocks of nuclei

17. Uses of Radioisotopes • Tracers • Easy to detect • Different isotopes have similar chemical and physical properties • Physical, chemical, or biological processes • Agriculture • Induce heritable genetic alterations – mutations • Preservative • Destroys microorganisms with little change to taste or appearance of the food Nuclear Medicine • Used for two purposes • Therapeutic – treat or cure disease using radiation • Diagnostic – obtain information about patient’s health

18. Radiation Therapy • Radiation most lethal to dividing cells • Makes some forms of cancer susceptible • Try to destroy cancer cells before too much damage to healthy cells • Direct radiation at cancer cells • Gives rise to side effects Diagnostic Uses • Many different isotopes used • See Table 4.6 • Can measure specific things • Iodine-131 to locate tumors in thyroid • Selenium-75 to look at pancreas • Gadolinium-153 to determine bone mineralization

19. Imaging • Positron emission tomography (PET) • Uses an isotope that emits a positron • Observe amount of radiation released Penetrating Power of Radiation • The more mass the particle has, the less penetrating it is • The faster the particle is, the more penetrating it is

20. Prevent Radiation Damage • To minimize damage • Stay a distance from radioactive sources • Use shielding; need more with more penetrating forms of radiation

21. Solution First write the nuclear equation 15 8 0 +1 O e + ? The nucleon number does not change, but the atomic number becomes 8 – 1, or 7; and so the new product is nitrogen-15: 15 8 0 +1 15 7 O N e + Exercise 4.6 Phosphorus-30 is a positron-emitting radioisotope suitable for use in PET scans. What new element is formed when phosphorus-30 decays? Example4.6 One of the isotopes used for PET scans is oxygen-15, a positron emitter. What new element is formed when oxygen-15 decays?

22. Energy from Nucleus • E = mc2 • Lose mass, gain energy • For chemical reactions, mass changes are not measurable • For nuclear reactions, mass changes may be measurable

23. Binding Energy • Holds protons and neutrons together in the nucleus • The higher the binding energy, the more stable the element

24. Nuclear Fission • “Splitting the atom” • Break a large nucleus into smaller nuclei

25. Nuclear Chain Reaction • Neutrons from one fission event split further atoms • Only certain isotopes, fissile isotopes, undergo nuclear chain reactions

26. Manhattan Project • How to sustain the nuclear reaction? • How to enrich uranium to >90% 235U? • Only 0.7% natural abundance • How to make 239Pu (another fissile isotope)? • How to make a nuclear fission bomb? Radioactive Fallout • Nuclear bomb detonated; radioactive materials may rain down miles away and days later • Some may be unreacted U or Pu • Radioactive isotopes produced during the explosion

27. Nuclear Power Plants • Provide ~20% U.S. electricity • France >70% • Slow controlled release of energy • Need 2.5–3.5% 235U • Problem with disposal of radioactive waste Nuclear Fusion • Reaction takes smaller nuclei and builds larger ones • Also called thermonuclear reactions • Releases tremendous amounts of energy • 1 g of H would release same as 20 tons of coal