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NUCLEAR CHEMISTRY

NUCLEAR CHEMISTRY. By: Stephanie Chen and Stephanie Ng. Radioactivity. One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie (1876-1934).

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NUCLEAR CHEMISTRY

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  1. NUCLEAR CHEMISTRY By: Stephanie Chen and Stephanie Ng

  2. Radioactivity • One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work ofMarie Curie(1876-1934). • She discoveredradioactivity, the spontaneous disintegration of some elements into smaller pieces.

  3. Nuclear Reactions vs. Normal Chemical Changes • Nuclear reactions involve the nucleus • The nucleus opens, and protons and neutrons are rearranged • The opening of the nucleus releases a tremendous amount of energy that holds the nucleus together – called binding energy • “Normal” Chemical Reactions involve electrons, not protons and neutrons

  4. 23.1

  5. Types of Radiation • Alpha (ά) – a positively charged (+2) helium isotope - we usually ignore the charge because it involves electrons, not protons and neutrons • Beta (β) – an electron • Gamma (γ) – pure energy; called a ray rather than a particle

  6. Other Nuclear Particles • Neutron • Positron – a positive electron • Proton – usually referred to as hydrogen-1 • Any other elemental isotope

  7. Penetrating Ability

  8. A X Mass Number Element Symbol Z Atomic Number a particle proton neutron electron positron or or 1H 1p 0b 0e or 1 1 +1 1n 0e +1 0b 0 -1 -1 4a 4He or 2 2 Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons A 1 1 0 0 4 1 0 -1 +1 2 Z 23.1

  9. + + + + + 2 + 2 1n 1n 1n 1n 96 96 0 0 0 0 Rb Rb 37 37 138 235 138 235 Cs Cs U U 92 55 92 55 Balancing Nuclear Equations • Conserve mass number (A). The sum of protons plus neutrons in the products must equal the sum of protons plus neutrons in the reactants. 235 + 1 = 138 + 96 + 2x1 • Conserve atomic number (Z) or nuclear charge. The sum of nuclear charges in the products must equal the sum of nuclear charges in the reactants. 92 + 0 = 55 + 37 + 2x0 23.1

  10. or alpha particle - 212Po 4He + AX 2 Z 84 212Po 4He + 208Pb 2 82 84 4a 4He 2 2 212Po decays by alpha emission. Write the balanced nuclear equation for the decay of 212Po. 212 = 4 + A A = 208 84 = 2 + Z Z = 82 23.1

  11. 14C 14N + 0b + n 6 7 -1 40K 40Ca + 0b + n 19 20 -1 1n 1p + 0b + n 0 1 -1 11C 11B + 0b + n 6 5 +1 38K 38Ar + 0b + n 19 18 +1 1p 1n + 0b + n 1 0 +1 n and n have A = 0 and Z = 0 Nuclear Stability and Radioactive Decay Beta decay Decrease # of neutrons by 1 Increase # of protons by 1 Positron decay Increase # of neutrons by 1 Decrease # of protons by 1 23.2

  12. 37Ar + 0e 37Cl + n 18 17 -1 55Fe + 0e 55Mn + n 26 25 -1 1p + 0e 1n + n 1 0 -1 212Po 4He + 208Pb 2 82 84 252Cf 2125In + 21n 98 49 0 Nuclear Stability and Radioactive Decay Electron capture decay Increase # of neutrons by 1 Decrease # of protons by 1 Alpha decay Decrease # of neutrons by 2 Decrease # of protons by 2 Spontaneous fission 23.2

  13. Learning Check What radioactive isotope is produced in the following bombardment of boron? 10B + 4He 13N + 1n 5 2 7 0

  14. Write Nuclear Equations! Write the nuclear equation for the beta emitter Co-60. 60Co 0e + 60Ni27 -1 28

  15. Artificial Nuclear Reactions New elements or new isotopes of known elements are produced by bombarding an atom with a subatomic particle such as a proton or neutron -- or even a much heavier particle such as 4He and 11B. Reactions using neutrons are called g reactions because a g ray is usually emitted. Radioisotopes used in medicine are often made by g reactions.

  16. Artificial Nuclear Reactions Example of a g reaction is production of radioactive 31P for use in studies of P uptake in the body. 3115P + 10n ---> 3215P + g

  17. Transuranium Elements Elements beyond 92 (transuranium) made starting with an g reaction 23892U + 10n ---> 23992U + g 23992U ---> 23993Np + 0-1b 23993Np ---> 23994Pu + 0-1b

  18. Nuclear Stability • Certain numbers of neutrons and protons are extra stable • n or p = 2, 8, 20, 50, 82 and 126 • Like extra stable numbers of electrons in noble gases (e- = 2, 10, 18, 36, 54 and 86) • Nuclei with even numbers of both protons and neutrons are more stable than those with odd numbers of neutron and protons • All isotopes of the elements with atomic numbers higher than 83 are radioactive • All isotopes of Tc and Pm are radioactive 23.2

  19. Band of Stability and Radioactive Decay

  20. 3115P 21H, 63Li, 105B, 147N, 18073Ta 199F Stability of Nuclei • Out of > 300 stable isotopes: N Even Odd Z 157 52 Even Odd 50 5

  21. Half-Life • HALF-LIFE is the time that it takes for 1/2 a sample to decompose. • The rate of a nuclear transformation depends only on the “reactant” concentration.

  22. Half-Life Decay of 20.0 mg of 15O. What remains after 3 half-lives? After 5 half-lives?

  23. Kinetics of Radioactive Decay For each duration (half-life), one half of the substance decomposes. For example: Ra-234 has a half-life of 3.6 daysIf you start with 50 grams of Ra-234 After 3.6 days > 25 grams After 7.2 days > 12.5 grams After 10.8 days > 6.25 grams

  24. N daughter DN rate = - Dt DN - = lN Dt Ln 2 = l t½ Kinetics of Radioactive Decay rate = lN lnN = lnN0 - lt N = N0e(-lt) N = the number of atoms at time t N0 = the number of atoms at time t = 0 l is the decay constant (sometimes called k) k = 23.3

  25. [N] ln [N] Kinetics of Radioactive Decay [N] = [N]0exp(-lt) ln[N] = ln[N]0 - lt 23.3

  26. 14N + 1n 14C + 1H 0 7 6 1 14C 14N + 0b + n 6 7 -1 238U 206Pb + 8 4a + 6 0b 92 82 2 -1 Radiocarbon Dating t½ = 5730 years Uranium-238 Dating t½ = 4.51 x 109 years 23.3

  27. Learning Check! The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 31 hours?

  28. Nuclear Fission Fission is the splitting of atoms These are usually very large, so that they are not as stable Fission chain has three general steps: 1. Initiation. Reaction of a single atom starts the chain (e.g., 235U + neutron) 2. Propagation. 236U fission releases neutrons that initiate other fissions 3. Termination.

  29. Nuclear Fission

  30. 235U + 1n 90Sr + 143Xe + 31n + Energy 0 38 0 54 92 Nuclear Fission Energy = [mass 235U + mass n – (mass 90Sr + mass 143Xe + 3 x mass n )] x c2 Energy = 3.3 x 10-11J per 235U = 2.0 x 1013 J per mole 235U Combustion of 1 ton of coal = 5 x 107 J 23.5

  31. Representation of a fission process.

  32. Mass Defect • Some of the mass can be converted into energy • Shown by a very famous equation! E=mc2 Energy Mass Speed of light

  33. BE + 19F 91p + 101n 0 9 1 binding energy per nucleon = binding energy number of nucleons 2.37 x 10-11 J 19 nucleons = Nuclear binding energy (BE) is the energy required to break up a nucleus into its component protons and neutrons. E = mc2 BE = 9 x (p mass) + 10 x (n mass) – 19F mass BE (amu) = 9 x 1.007825 + 10 x 1.008665 – 18.9984 BE = 0.1587 amu 1 amu = 1.49 x 10-10 J BE = 2.37 x 10-11J = 1.25 x 10-12 J 23.2

  34. Nuclear binding energy per nucleon vs Mass number nuclear binding energy nucleon nuclear stability 23.2

  35. Non-critical Critical Nuclear Fission Nuclear chain reaction is a self-sustaining sequence of nuclear fission reactions. The minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction is the critical mass. 23.5

  36. Nuclear Fusion Fusion small nuclei combine 2H + 3H 4He + 1n + 1 1 2 0 Occurs in the sun and other stars Energy

  37. Nuclear Fusion Fusion • Excessive heat can not be contained • Attempts at “cold” fusion have FAILED. • “Hot” fusion is difficult to contain

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