Chapter 21 Nuclear Chemistry

Transcription

1 Chapter 21 Nuclear Chemistry The chemical energy: we have discussed thus far originates in the making and breaking of chemical bonds, which result from the interaction of electrons between atoms. In this chapter Reactions in which the nuclei of atoms undergo change Pearson Education, Inc.

2 Energy: Chemical vs. Nuclear Chemical energy: is associated with making and breaking chemical bonds. Nuclear energy: is enormous in comparison. is due to changes in the nucleus of atoms changing them into different atoms. 13% of worldwide energy use comes from nuclear energy.

3 Nucleus is composed of nucleons ( 核子 ) Protons (+) and neutrons (0) has very small volume compared to volume of the atom is essentially entire mass of atom is very dense Protons repel each other by electric forces. (+) (+) like charges repel Neutrons have no charge, so they do not repel each other The presence of neutrons helps hold the nucleus together. In large nuclei, far-apart neutrons are less effective in holding a nucleus together. less effective

4 Terminology Nucleons ( 核子 ) Nuclides ( 核種 ) Radionuclide Radioisotope ( 放射性同位素 ) Nuclear particles, neutrons and protons A nucleus containing a specific number of protons and neutrons Nuclides that are radioactive Atoms with same numbers of protons (atomic #) but different mass numbers Nuclear reactions Radioactive decay Nuclear force chemical changes that involve nuclear particles and radioactive decay nucleus of element spontaneously decomposes by emitting a nuclear particle, electron, positron ( 正子 ), or electromagnetic radiation strong force of attraction between nuclear particles

5 Radioactivity It is the spontaneous decay/disintegration of an unstable atomic nucleus. The emission of radiation (energy) and/or matter (protons, neutrons, smaller isotopes, beta and/or alpha particles) Particles are ejected from the nucleus. Occurs as a result of an unstable ratio of neutrons to protons (N/P ratio) in an atom s nucleus. All nuclides with 84 or more protons are radioactive

6 Radioactive rays An unstable nucleus is transformed into a more stable one that has less energy. The emitted radiation is carrier of the excess energy. can ionize matter, which causes uncharged matter to become charged and can be detected using a Geiger counter ( 蓋格計數器 ) or an electroscope have high energy can penetrate matter cause phosphorescent ( 磷光 ) chemicals to glow

7 Nuclear equation We use nuclear equations to show how these nuclear reactions occur. In chemical equations, atoms and charges need to balance. In nuclear equations, atomic number and mass number need to balance. This is a way of balancing charge (atomic number) and mass (mass number) on an atomic scale U 234 Th 4 He 90 2 mass number 238=234+4 atomic number 92=90+2

8 Types of Radioactive Decay Alpha (α) decay/radiation Beta (β) decay/radiation Gamma (γ) decay/radiation Positron emission Electron capture

9 Alpha Decay Alpha decay is the loss of an α-particle α-particle: (He-4 nucleus, two protons and two neutrons): 4 He U Th He The equation balances: atomic number: 92 = mass number: 238 =

10 Beta Decay Beta decay is the loss of a β-particle β-particle: a high-speed electron emitted by the nucleus: 0 1β or 0 e 1 Win one proton 131 I 131 Xe e 0 Balancing: atomic number: 53 = 54 + ( 1) mass number: 131 =

11 Gamma (γ) decay Gamma emission is the loss of a γ-ray, which is high-energy radiation that almost always accompanies the loss of a nuclear particle. Are high-frequency electromagnetic radiation emitted when a nucleus in an excited state moves to a lower energy state. Are more harmful than alpha or beta particles and the MOST penetrating because they have no mass or charge, just pure energy. 0 γ 0

12 Positron Emission Some nuclei decay by emitting a positron ( 正子 ), e a particle that has the same mass as, but an opposite charge to, that of an electron: 0 1 proton morphs into a neutron 11 C 6 11 B e 1 Balancing: atomic number: 6 = mass number: 11 =

13 Electron Capture (K-Capture) An electron from the surrounding electron cloud is absorbed into the nucleus during electron capture. Atomic number decreases by one. Mass number stays the same. 81 Rb 37 + e Kr 36 Balancing: atomic number: 37 + ( 1) = 36 mass number: = 81

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15 Sample Exercise 21.1 Predicting the Product of a Nuclear Reaction What product is formed when radium-226 undergoes alpha decay?

16 Nuclear Stability 12 C 13 C 14 C WHY??? stable unstable Any atom with more than one proton (anything but H) will have repulsions between the protons in the nucleus. Strong nuclear force helps keep the nucleus together. (~~because of neutrons) Neutrons play a key role stabilizing the nucleus, so the ratio of neutrons to protons (N/P ratio) is an important factor.

17 Neutron Proton Ratios Neutrons (N) vs. Protons (Z, P) For smaller nuclei (Z 20), stable nuclei have a neutron-to-proton ratio close to 1:1. As nuclei get larger, it takes a larger number of neutrons to stabilize the nucleus. As N/P increases to 1.5 above Z = 20, neutrons become more abundant with increasing P due to repulsion by so many protons. No stable nuclides exist with Z >= 84.

18 Nuclei above this belt N>P, have too many neutrons tend to decay by emitting β particles. Nuclei with such large atomic numbers tend to decay by α emission. Nuclei below the belt have too many protons (+) tend to become more stable by positron emission or Blue dots: stable (nonradioactive) isotopes Blue dots region: belt of stability electron capture.

19 Radioactive Decay Chain Some 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). The decay processes are either α emission or β emission. α emission: mass number atomic number (proton) β emission: mass number - atomic number (proton)

20 Sample Exercise 21.3 Predicting Modes of Nuclear Decay Predict the mode of decay of (a) carbon-14, (b) xenon-118. Carbon is element 6. Thus, carbon-14 has 6 protons and 14 6 = 8 neutrons, giving it a N/P ratio of=1.25. Elements with Z < 20 normally, above the belt of stability (left) β emission Xenon is element 54. Xenon-118 has 54 protons and = 64 neutrons, giving it a N/P ratio=1.18, below the belt of stability (left) positron emission or electron capture

21 Stable Nuclei Besides the N/Z ratio, the actual numbers of protons and neutrons affect stability. Magic numbers of 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons result in more stable nuclides. Nuclei with an even number of protons and neutrons tend to be more stable than those with odd numbers.

22 Nuclear Transmutations ( 轉換 ) We have examined nuclear reactions in which decays spontaneously. Nuclear transmutations: Nucleus is struck by a nuclide (α particle) or a neutron ( ). Particle accelerators ( atom smashers ) are enormous, having circular tracks with radii that are miles long. 重離子碰撞器 (Relativistic Heavy Ion Collider:RHIC)

23 Use of neutrons: Other Nuclear Transmutations Most synthetic isotopes used in medicine are prepared by bombarding neutrons at a particle, which won t repel the neutral particle. cancer radiation therapy, is produced by neutron capture Transuranium elements ( 超鈾元素 ): Elements immediately after uranium ( 鈾 ) were discovered by bombarding isotopes with neutrons. Larger elements (atomic number higher than 110) were made by colliding large atoms with nuclei of light elements with high energy.

24 Writing Nuclear Equations for Nuclear Transmutations 1) Nuclear equations that represent nuclear transmutations are written two ways: 2) Sample Exercise 21.4 Writing a Balanced Nuclear Equation

25 Kinetics of Radioactive Decay Radioactive decay is a 1 st order kinetics reaction. 一級反應的半衰期與反應物的起始濃度無關 Rate = kn N = number of radioactive nuclei K = the rate constant N t = number of radioactive nuclei at time, t N 0 = initial number of radioactive nuclei The half-life for 1 st order kinetics is The shorter the half-life, the more nuclei decay. The rate of radioactive change is not affected by temperature. The radioactivity is not a chemical reaction! Radioactive atoms cannot be rendered harmless by chemical reaction.

26 Sample Exercise 21.5 Calculation Involving Half-Lives The half-life of cobalt-60 is 5.27 yr. How much of a mg sample of cobalt-60 is left after yr? K=0.693/5.27= ln(nt/1 mg)= *15.81 yr

27 How to know the age of objects? Radiometric Dating C-14 Carbon dating works: the half-life of C-14 is 5700 yr. It is limited to objects up to about 50,000 yr old; after this time there is too little radioactivity left to measure. Other isotopes can be used (U-238:Pb-206 in rock).

28 Measuring Radioactivity: Units Activity is the rate at which a sample decays = kn k: decay content, N: number of nuclei The units used to measure activity are as follows: Becquerel (Bq): one disintegration ( 衰變 ) per second Curie (Ci): disintegrations per second, which is the rate of decay of 1 g of radium (1g 鐳的放射量 ). 1 Ci = Bq

29 21.6 Calculating the Age of Objects Using Radioactive Decay A rock contains mg of lead (Pb)-206 for every milligram of uranium (U)-238. The half-life for the decay of uranium-238 to lead-206 is yr. How old is the rock? How many weight of U transfer to Pb N t N 0

30 21.7 Calculations Involving Radioactive Decay and Time If we start with g of strontium( 鍶 )-90, g will remain after 2.00 yr. (a) What is the half-life of strontium-90? (b) How much strontium-90 will remain after 5.00 yr? (c) What is the initial activity of the sample in becquerels and curies? = -k*2 yr K = /yr Q: Would doubling the mass of a radioactive sample change the half-life for the radioactive decay? A: No

31 If we start with g of strontium( 鍶 )-90, g will remain after 2.00 yr. (a) What is the half-life of strontium-90? (b) How much strontium-90 will remain after 5.00 yr? (c) What is the initial activity of the sample in becquerels and curies? The number of disintegrations per atom per second is given by the decay constant, k: For the total number of disintegrations per second R=k*N the total number of atoms: N Bq Q: If the size (1g to 2g) of a radioactive sample is doubled, what happens to the activity of the sample in Bq? A: It is doubled.

32 How to Measuring Radioactivity Film badges ( 膠片式輻射計量器 ) Geiger counter ( 蓋格計數器.) Phosphors (scintillation counters) ( 磷光劑閃爍計數器 )

33 Film Badges Radioactivity was first discovered by Henri Becquerel because it fogged up a photographic plate. Film has been used to detect radioactivity since more exposure to radioactivity means darker spots on the developed film. Film badges are used by people who work with radioactivity to measure their own exposure over time. Gamma ray

34 Phosphors Some substances absorb radioactivity and emit light. They are called phosphors. An instrument commonly used to measure the amount of light emitted by a phosphor is a scintillation counter. It converts the light to an electronic response for measurement. ZnS responds to α particals. NaI responds to X ray or γ ray

35 Radiotracers Radiotracers are radioisotopes used to study a chemical reaction. An element can be followed through a reaction to determine its path and better understand the mechanism of a chemical reaction. The use of the carbon-14 label provides that carbon dioxide is chemically converted to glucose in plants. Oxygen-18 show that the O 2 produced during photosynthesis comes from water, not carbon dioxide.

36 Medical Application of Radiotracers Radiotracers have found wide diagnostic use in medicine. Radioisotopes are administered to a patient (usually intravenously) and followed. Certain elements collect more in certain tissues, so an organ or tissue type can be studied based on where the radioactivity collects.

37 Positron Emission Tomography (PET Scan) A compound labeled with a positron emitter is injected into a patient. Blood flow, oxygen and glucose metabolism, and other biological functions can be studied. Labeled glucose (F-18-FDG) is used to study the brain, as seen in the figure to the right.

38 Why are the energies associated with nuclear reactions so large? Energy in Nuclear Reactions Einstein s famous equation, E = mc 2, relates directly to the calculation of this energy. E: energy M: mass C: the speed of light, *10 8 m/s

39 238 U 92 : amu 234 Th 90 4 He 2 : amu : amu There is a tremendous amount of energy stored in nuclei. Note: the negative sign means heat is released.

40 Where does this energy come from? Mass Defect The masses of nuclei are always less than those of the individual parts (protons+neutron). This mass difference is called the mass defect. The energy needed to separate a nucleus into its nucleons is called the nuclear binding energy. <

41 Effects of Nuclear Binding Energy on Nuclear Processes Heavy nuclei gain stability and give off energy when they split into two smaller nuclei. This is fission ( 核分裂 ). Lighter nuclei emit great amounts of energy by being combined in fusion ( 核融合 ). Fission and Fusion are exothermic process.

42 Nuclear Fission( 核分裂 ) Commercial nuclear power plants use fission. Heavy nuclei can split in many ways. The equations below show two ways U-235 can split after bombardment with a neutron. bombardment

43 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. This process continues in what we call a nuclear chain reaction. (neutrons trigger the reactions)

44 Little Boy A conventional explosive is used to bring two subcritical masses together to form a supercritical mass. The minimum mass that must be present for a chain reaction to be sustained is called the critical mass. the critical mass of U-235 is about 50 kg for a bare sphere of the metal If more than critical mass is present (supercritical mass), an explosion will occur. Weapons were created by causing smaller amounts to be forced together to create this mass.

45 Nuclear Reactors The reactor core consists of fuel rods, control rods, moderators/coolant. The fissionable material is stored in long tubes, called fuel rods, arranged in a matrix. The control rods block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass. The rods are placed in a material (water, D 2 O, graphite, Li, Be, biphenyl, and terphenyl) to slow down the ejected neutrons, called a moderator.

46 Nuclear Fusion: Smaller to Bigger When small atoms are combined, much energy is released. If it were possible to easily produce energy by this method, it would be a preferred source of energy. However, extremely high temperatures and pressures are needed to cause nuclei to fuse. This was achieved using an atomic bomb ( 核分裂 ) to initiate fusion in a hydrogen bomb. Obviously, this is not an acceptable approach to producing energy.

47 Radiation in the Environment We are constantly exposed to radiation. Ionizing radiation is more harmful to living systems than nonionizing radiation (such as radiofrequency electromagnetic radiation). Since most living tissue is ~70% water, ionizing radiation is that which causes water to ionize. This creates unstable, very reactive OH radicals, which result in much cell damage. hydroxyl radical attack biomolecules

48 Damage to Cells The damage to cells depends on the type of radioactivity, the length of exposure, and whether the source is inside or outside the body. Outside the body, gamma rays are most dangerous. Inside the body, alpha radiation can cause most harm.

49 Exposure We are constantly exposed to radiation. What amount is safe? Setting standards for safety is difficult. Low-level, long-term exposure can cause health issues. Damage to the growth-regulation mechanism of cells results in cancer.

50 Measuring Radiation Exposure: Units The curie (Ci) is an exposure of events per second. No matter the kind of radiation The gray (Gy) measures the amount of energy absorbed by body tissue from radiation. 1 Gy = 1 J/kg body tissue Rad (for radiation absorbed dose): absorption of 0.01 J/body weight kg of tissue (100 rad = 1 Gy) A correction factor is used to account for a number of factors that affect the result of the exposure this biological effectiveness factor is the RBE, and the result is the dose in REMs. Rads RBE = REMs The RBE is approximately 1 for γ and β radiation, and 10 for α radiation. REM = roentgen equivalent man 1 REM =0.01Sv (sievert),1 Rad ( 拉德 ) =0.01Gy, 1 Gy = 1 J/kg

51 If a 50-kg person is uniformly irradiated by 0.10-J alpha radiation, what is the absorbed dosage in rad and the effective dosage in REM? REMs = Rads RBE 1 Rad =0.01Gy, 1 Gy = 1 J/kg RBE is approximately 10 for alpha radiation 0.1 J/kg=10 Rad REM= 10*10=100 REMs

52 Short-Term Exposure 600 REM is fatal to most humans. Average exposure per year is about 360 mrem. The amount of danger to humans of radiation is measured in the unit REMs. More energetic radiation has a larger effect. More ionizing radiation has a larger effect. --- Alpha > Beta > Gamma Less ionizing radiation penetrates human tissue more deeply. --- Gamma >> Beta > Alpha Inside the body, alpha radiation can cause most harm. Outside the body, gamma rays are most dangerous.