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

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Presentation on theme: "Nuclear Chemistry Chapter 21 Nuclear Chemistry Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten."— Presentation transcript:

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

2 Nuclear Chemistry February 3 Nuclear chemistry HW 1,2,3,7,11,13,17,19,27,29 for tomorrow 31 to35 odd, 41,57,59,61

3 Nuclear Chemistry The Nucleus Remember that the nucleus is comprised of the two nucleons, protons and neutrons. The number of protons is the atomic number. The number of protons and neutrons together is effectively the mass of the atom.

4 Nuclear Chemistry Isotopes Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms. There are three naturally occurring isotopes of uranium:  Uranium-234  Uranium-235  Uranium-238

5 Nuclear Chemistry

6 Nuclear Chemistry Radioactivity It is not uncommon for some nuclides of an element to be unstable, or radioactive. We refer to these as radionuclides. There are several ways radionuclides can decay into a different nuclide.

7 Nuclear Chemistry Types of Radioactive Decay

8 Nuclear Chemistry

9 Nuclear Chemistry SeparationAlphaBetaGamma.MO VSeparationAlphaBetaGamma.MO V Separation of Radiation

10 Nuclear Chemistry

11 Nuclear Chemistry

12 Nuclear Chemistry

13 Nuclear Chemistry Nuclear Reactions The chemical properties of the nucleus are independent of the state of chemical combination of the atom. In writing nuclear equations we are not concerned with the chemical form of the atom in which the nucleus resides. It makes no difference if the atom is as an element or a compound. Mass and charges MUST BE BALANCED!!!

14 Nuclear Chemistry Alpha Decay: Loss of an  -particle (a helium nucleus) He 4242 U  Th He

15 Nuclear Chemistry Alpha Decay Mass changes by 4 The remaining fragment has 2 less protons Alpha radiation is the less penetrating of all the nuclear radiation (it is the most massive one!)

16 Nuclear Chemistry Beta Decay: Loss of a  -particle (a high energy electron)  0−10−1 e 0−10−1 or I Xe  + e 0−10−1

17 Nuclear Chemistry Beta Decay Involves the conversion of a neutron in the nucleus into a proton and an electron. Beta radiation has high energies, can travel up to 300 cm in air. Can penetrate the skin

18 Nuclear Chemistry Beta decay Write the reaction of decay for C-14

19 Nuclear Chemistry Gamma Emission: Loss of a  -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)  0000

20 Nuclear Chemistry Positron Emission: Loss of a positron ( particle with same mass, but opposite charge than an electron) e C 11 6  B e 0101

21 Nuclear Chemistry Positron emission Involves the conversion of a proton to a neutron emitting a positron. The atomic number decreases by one, mass number remains the same.

22 Nuclear Chemistry Electron Capture (K-Capture) Capture by the nucleus of an electron from the electron cloud surrounding the nucleus.  As a result, a proton is transformed into a neutron. p e 0−10−1  n 1010

23 Nuclear Chemistry Electron capture Rb-81 Note that the electron goes in the side of the reactants. Electron gets consumed. Rb  Kr 81 36

24 Nuclear Chemistry Patterns of nuclear Stability Any element with more than one proton ( all but hydrogen) will have repulsions between the protons in the nucleus. A strong nuclear force helps keep the nucleus from flying apart.

25 Nuclear Chemistry Neutron-Proton Ratios Neutrons play a key role stabilizing the nucleus. The ratio of neutrons to protons is key to determine the stability of a nucleus.

26 Nuclear Chemistry Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

27 Nuclear Chemistry Neutron-Proton Ratios For smaller nuclei (Z  20) stable nuclei have a neutron-to-proton ratio close to 1:1.

28 Nuclear Chemistry Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the so- called belt of stability.

29 Nuclear Chemistry Stable Nuclei Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles. ( neutron becomes proton )

30 Nuclear Chemistry Above the belt of stability Beta particle emission Too many neutrons. The nucleus emits Beta particles, decreasing the neutrons and increasing the number of protons.

31 Nuclear Chemistry Stable Nuclei Nuclei below the belt have too many protons. They tend to become more stable by positron emission or electron capture (both lower the number of protons)

32 Nuclear Chemistry Stable Nuclei Elements with low atomic number are stable if # proton = # neutrons There are no stable nuclei with an atomic number greater than 83. These nuclei tend to decay by alpha emission.

33 Nuclear Chemistry Below the stability belt Increase the number of neutrons (by decreasing # protons) Positron emission more common in lighter nuclei. Electron capture common for heavier nuclei.

34 Nuclear Chemistry

35 Nuclear Chemistry Radioactive Series Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

36 Nuclear Chemistry Predicting modes of nuclear decay C-14 Xe-118 Pu-239 In-120

37 Nuclear Chemistry beta decay Positron emission or electron capture Alpha decay (too heavy, loses mass) Beta decay (ratio too low, gains protons)

38 Nuclear Chemistry MAGIC NUMBERS 2, 8, 20, 28, 50, or 82 Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons.

39 Nuclear Chemistry Some Trends Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.

40 Nuclear Chemistry Shell model of the nucleus Nucleons are described a residing in shells like the shells for electrons. The numbers 2,8,18,36,54,86 correspond to closed shells in nuclei. Evidence suggests that pair of protons and pairs of neutrons have special stability

41 Nuclear Chemistry Transmutations To change one element into another. Only possible in nuclear reactions never in a chemical reaction. In order to modify the nucleus huge amount of energy are involved. These reactions are carried in particle accelerators or in nuclear reactors

42 Nuclear Chemistry Nuclear transmutations Alpha particles have to move very fast to overcame electrostatic repulsions between them and the nucleus. Particle accelerators or smashers are used. They use magnetic fields to accelerate the particles.

43 Nuclear Chemistry ParticleParticle Accelerators (only for charged particles!) These particle accelerators are enormous, having circular tracks with radii that are miles long.

44 Nuclear Chemistry Cyclotron Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.

45 Nuclear Chemistry Neutrons Can not be accelerated. They do not need it either (no charge!). Neutrons are products of natural decay, natural radioactive materials or are expelled of an artificial transmutation. Some neutron capture reactions are carried out in nuclear reactors where nuclei can be bombarded with neutrons.

46 Nuclear Chemistry Representing artificial nuclear transmutations 14 N + 4 He  7 O + 1 H Target nucleus ( bombarding particle, ejected particle ) product nucleus 14 N (  p) 17 O Write the balanced nuclear equations summarized as followed: 16 O ( p,  27 Al (n,    a 

47 Nuclear Chemistry Measuring Radioactivity One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample. The ionizing radiation creates ions, which conduct a current that is detected by the instrument.

48 Nuclear Chemistry Mass defect The mass of the nucleus is always smaller than the masses of the individual particles added up. The difference is the mass defect. That small amount translate to huge amounts of energy  E = (  m) c 2 That energy is the Binding energy of the nucleus, and is the energy needed to separate the nucleus.

49 Nuclear Chemistry Energy in Nuclear Reactions For example, the mass change for the decay of 1 mol of uranium-238 is − g. The change in energy,  E, is then  E = (  m) c 2  E = (−4.6  10 −6 kg)(3.00  10 8 m/s) 2  E = −4.1  J This amount is 50,000 times greater than the combustion of 1 mol of CH 4

50 Nuclear Chemistry Types of nuclear reactions fission and fusion The larger the binding energies, the more stable the nucleus is toward decomposition. Heavy nuclei gain stability (and give off energy) if they are fragmented into smaller nuclei. (FISSION)

51 Nuclear Chemistry Even greater amounts of energy are released if very light nuclei are combined or fused together. (FUSION)

52 Nuclear Chemistry Nuclear Fission How does one tap all that energy? Nuclear fission is the type of reaction carried out in nuclear reactors.

53 Nuclear Chemistry Nuclear Fission Bombardment of the radioactive nuclide with a neutron starts the process. Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

54 Nuclear Chemistry Nuclear Fission This process continues in what we call a nuclear chain reaction.

55 Nuclear Chemistry Nuclear Fission If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.

56 Nuclear Chemistry Nuclear Fission Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.

57 Nuclear Chemistry Controlled vs Uncontrolled nuclear reaction Controlled reactions: inside a nuclear power plant Uncontrolled reaction: nuclear bomb

58 Nuclear Chemistry Nuclear Reactors In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.

59 Nuclear Chemistry Nuclear Reactors The reaction is kept in check by the use of control rods. These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.

60 Nuclear Chemistry FUSION Combining small nucleii to form a larger one. Require millions of K of temperature

61 Nuclear Chemistry Fusion 1 H + 1 H  2 H + 1 e + energy 1 H + 2 H  3 He + energy 3 He + 3 He  4 He H + energy Reaction that occurs in the sun Temperature 10 7 K Heavier elements are synthesized in hotter stars 10 8 K using Carbon as fuel

62 Nuclear Chemistry Nuclear Fusion Fusion would be a superior method of generating power.  The good news is that the products of the reaction are not radioactive.  The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.

63 Nuclear Chemistry Nuclear Fusion (thermonuclear reactions) Tokamak apparati like the one shown at the right show promise for carrying out these reactions. They use magnetic fields to heat the material. 3 million K degrees were reached inside but is not enough to begin fusion which requires 40 million K

64 Nuclear Chemistry Rates of radioactive decay rate = k N N is the number of radioactive nuclei Activity: rate at which a sample decays. Expressed in disintegrations per unit time. Becquerel (Bq) SI unit : one nuclear disintegration per second. Curie (Ci) 3.7x10 10 disintegrations per second, the rate of decay of 1g of Ra

65 Nuclear Chemistry RADIOACTIVE DECAY As a radioactive sample decays, the amount of radiation emanating for the sample decays as well. After one half life, half of the emanations!

66 Nuclear Chemistry Half-Life Half-life is defined as the time required for one-half of a reactant to react. Because [A] at t 1/2 is one-half of the original [A], [A] t = 0.5 [A] 0.

67 Nuclear Chemistry RADIOACTIVE DECAY Is a first order process. Its rate is proportional to the number of radioactive nuclei N in the sample rate= k N N0NtN0Nt ln = kt Time elapsed =t k is the decay constant N 0 is the original amount N t is the amount of sample at time t = kt 1/2

68 Nuclear Chemistry Half life The half life of a reaction is useful to describe how fast it occurs. For a first order reaction (like nuclear decay!) it does not depend on the initial concentration of the reactants. HALF LIFE IS CONSTANT FOR A FIRST ORDER REACTION

69 Nuclear Chemistry Half Life Decay of 10.0 g sample of Sr-90 t 1/2 = 28.8 y

70 Nuclear Chemistry Problem 1 The half life of 210 Pb= 25 y 1) How much left of a sample of 50 mg will remain after 100 y? 2) Find number of half lives 3) Find fraction left

71 Nuclear Chemistry g 2- 4 half lives 3- 1/16

72 Nuclear Chemistry Problem 2 How many years will take for 50mg of 210 Pb to decay to 5 mg? Half life of 210 Pb= 25 y

73 Nuclear Chemistry 83 years

74 Nuclear Chemistry Problem 3 90 % of a radioisotope disintegrates in 36 hs. What is the half life?

75 Nuclear Chemistry Problem g of Sr-90 remains after 2 y from a 1.000g sample. a) find the half life b) how much will remain after 5 y?

76 Nuclear Chemistry Half life = 28.8 years Amount left (No) = 0.89 g

77 Nuclear Chemistry Radioactive Dating A rock contains.257 mg of Pb-206 for every mg of U-238. T 1/2 = 4.5 x 10 9 y How old is the rock?

78 Nuclear Chemistry N o = we will assume that all the Pb-206 that is now present will come from the original U, plus the U that is still present (check the answer in textbook!)

79 Nuclear Chemistry Calculating half life: If 87.5 % of a sample of I-131 decays in 24 days, what is the half life of the I-131?


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