Fundamentals of General, Organic, and Biological Chemistry 7th Edition Chapter Eleven Nuclear Chemistry

Chapter 11 Goals Topics and Goals for the Chapter: What is a nuclear reaction, and how are equations for nuclear reactions balanced? Understand the differences between nuclear reactions and chemical reactions. Be able to define nucleon and nuclide and write nuclide symbols for an isotope. Be able to write and balance equations for nuclear reactions. What are the different kinds of radioactivity? Be able to list characteristics of three common kinds of radiation – α, β and γ (alpha, beta and gamma). Be able to name and discuss the five common types of nuclear decay. 2013 Pearson Education, Inc. Chapter 11

Chapter 11 Goals Cont. How are the rates of nuclear reactions expressed? Be able to define and discuss the concept of radioactive half-life and calculate the quantity of a radioisotope remaining after a given number of half-lives. What is ionizing radiation? Be able to describe the properties of the different types of ionizing radiation and their potential for harm to living tissue. How is radioactivity measured? Be able to describe the common units for measuring radiation. What is transmutation? Be able to define transmutation, explain nuclear bombardment and balance equations for nuclear bombardment reactions. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 3 3

Chapter 11 Goals Cont. What are nuclear fission and fusion? Be able to define and explain nuclear fission and nuclear fusion. Be familiar with the ways in which nuclear reactions are utilized for energy production and nuclear weapons. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 4 4

11.1 Nuclear Reactions Review sections 3.1 – 3.3! Atomic Number Number of protons in the nucleus. Identifies the element. Mass Number Gives the total number of nucleons - A general term for both protons (p) and neutrons (n). Isotopes Atoms with identical atomic numbers but different mass numbers. Nucleus of an isotope - nuclide 2013 Pearson Education, Inc. Chapter 11

Isotopes of carbon Carbon has 13 isotopes. Commonly occurring 12C and 13C are stable indefinitely Other 11 unstable - undergo spontaneous nuclear reactions. 14C is produced in small amounts in the upper atmosphere. The other 10 unstable isotopes are produced artificially. The most common isotope of carbon, 12C, has 12 nucleons: 6 protons and 6 neutrons: 2013 Pearson Education, Inc. Chapter 11

Nuclear Reactions Nuclear reactions are very different from chemical reactions. Nuclear Reaction: A reaction that changes an atomic nucleus, usually causing the change of one element into another. A chemical reaction never changes the nucleus. Different isotopes of an element have essentially the same behavior in chemical reactions but often have completely different behavior in nuclear reactions. The rate of a nuclear reaction is unaffected by a change in temperature or pressure (within the range found on Earth) or by the addition of a catalyst. The nuclear reaction of an atom is essentially the same whether it is in a chemical compound or in an uncombined, elemental form. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 7

Nuclear Reactions The energy change accompanying a nuclear reaction can be up to several million times greater than that accompanying a chemical reaction. Example: Nuclear transformation of 1.0g of uranium-235 Releases 3.4x108 kcal of energy Combustion of 1.0g methane Releases 12 kcal of energy Example of a nuclear reaction: 146C → 147N + o-1e (electron) o-1e = electron 0 = mass, -1 = charge 2013 Pearson Education, Inc. Chapter 11 Chapter 11 8

11.2 The Discovery and Nature of Radioactivity In 1896, the French physicist Henri Becquerel noticed a uranium-containing mineral exposed a photographic plate that had been wrapped in paper. Marie and Pierre Curie investigated this new phenomenon, which they termed radioactivity: The spontaneous emission of radiation from a nucleus. Ernest Rutherford established that there were at least two types of radiation, which he named alpha (α)and beta (β). Shortly thereafter, a third type of radiation was found and named for the third Greek letter, gamma (γ). 2013 Pearson Education, Inc. Chapter 11

Differences in Types of Radiation Differences in Charges: When passed between two charged plates: Alpha rays, helium nuclei (He+2), bend toward the negative plate because they have a positive charge. Beta rays, electrons (e-), bend toward the positive plate because they have a negative charge. Gamma rays, photons (γ), do not bend toward either plate because they have no charge. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 10

Differences in Types of Radiation Continued Differences in Mass: Alpha rays = 4 amu Beta rays = 1/1823 amu Gamma rays = 0 mass 2013 Pearson Education, Inc. Chapter 11 Chapter 11 11

Differences in Types of Radiation Continued Differences in Speed and Penetration: Alpha rays move about 10% of the speed of light. α particles can be stopped by a few sheets of paper or by the top layer of skin. Beta rays move at up to about 90% of the speed of light and have about 100 times the penetrating power of α particles. A block of wood or heavy clothing is necessary to stop β rays. Gamma rays move at the speed of light and have about 1000 times the penetrating power of α rays. A lead block several inches thick is needed to stop γ rays. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 12

2013 Pearson Education, Inc. Chapter 11 Figure: 11-01UN00T01 Title: Characteristics of alpha, beta, and gamma radiation Caption: Comparing characteristics of the various types of radiation. Notes: Keywords: alpha, beta, gamma, radiation 2013 Pearson Education, Inc. Chapter 11

11.3 Stable and Unstable Isotopes Every element in the periodic table has at least one radioactive isotope, or radioisotope. More than 3300 radioisotopes are known. Radioactivity is the result of having unstable nuclei. Radiation is emitted when an unstable radioactive nucleus, or radionuclide, spontaneously changes into a more stable one. There are only 264 stable isotopes among all the elements. All isotopes of elements with atomic numbers higher than that of bismuth (83) are radioactive. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 14

Belt of Stability For elements in the first few rows of the periodic table: Stability Neutron-to-Proton ratio = 1:1 2013 Pearson Education, Inc. Chapter 11 Chapter 11 15

Belt of Stability As elements get heavier, the number of neutrons relative to protons in stable nuclei increases. Stability Neutron-to-Proton ratio ≠ 1:1 # Neutrons > # Protons Example: Lead-208 The most abundant stable isotope of lead, has 126 neutrons and 82 protons in its nuclei. 2013 Pearson Education, Inc. Chapter 11

The majority of isotopes are artificially generated Referred to as artificial radioisotopes. Not found in nature. All isotopes of elements heavier than uranium are artificial. Because isotopes have the same chemical properties, radioisotopes can be used as tracers. Example: Body imaging 2013 Pearson Education, Inc. Chapter 11

11.4 Nuclear Decay Nuclear or radioactive decay The spontaneous emission of a particle from an unstable nucleus. Transmutation The change of one element into another. The result of nuclear decay. Equations for nuclear reactions. Not balanced in the usual chemical sense. The kinds of atoms are not the same on both sides of the arrow. Balanced when the number of nucleons and the sums of the charges are the same on both sides. Nuclear Decay Radioactive element → New Element + Emitted Particle 2013 Pearson Education, Inc. Chapter 11

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Nuclear Decay – Alpha Emission Figure: 11-03 Title: Alpha emission illustrated Caption: Alpha emission. Emission of an alpha particle from an atom of uranium-238 produces an atom of thorium-234. Notes: Alpha emission is the emission of an alpha particle from an atom. Keywords: nuclear decay, transmutation, alpha radiation, nuclear reaction 2013 Pearson Education, Inc. Chapter 11

During alpha emission, the nucleus loses two protons and two neutrons. Example: Emission of an α particle from an atom of uranium-238 produces an atom of thorium-234 – Identity of atom changes. The equation is balanced when the sum of the nucleons and charges are equal on both sides of equation. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 21

Nuclear Decay – Beta Emission Beta (β) emission involves the decomposition of a neutron to yield a proton and an electron. 10n → 11p + 0-1e Electron comes from the nucleus, not orbitals surrounding nucleus. Example: Iodine-131, a radioisotope used in detecting thyroid problems, undergoes nuclear decay by β emission to yield xenon-131. Atomic number increases by 1. Mass number remains unchanged. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 22

Nuclear Decay – Gamma Emission Gamma (γ) emission involves no change in mass or atomic number. γ rays are high energy electromagnetic waves. 00γ Can occur alone but normally accompanies α or β emission. High penetration makes them the most dangerous to humans but useful in medical applications. Example: Cobalt-60 used in cancer therapy. 6027Co → 6028Ni + 0-1e + 00γ 2013 Pearson Education, Inc. Chapter 11 Chapter 11 23

Nuclear Decay – Positron Emission Positron emission involves the conversion of a proton in the nucleus into a neutron plus an ejected positron. A positron has the same mass as an electron but a positive charge. 01e or β+ 11p → 10n + 01e Example Potassium-40 undergoes positron emission to yield argon-40. Decrease in atomic number No change in mass number 2013 Pearson Education, Inc. Chapter 11 Chapter 11 24

Nuclear Decay – Electron Capture Electron capture, symbolized E.C., is a process in which the nucleus captures an inner-shell electron from the surrounding electron cloud, thereby converting a proton into a neutron. Example: The conversion of mercury- 197 into gold-197. Mass number unchanged. Atomic number decreases by 1. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 25

2013 Pearson Education, Inc. Chapter 11 Figure: 11-03UN04T02 Title: A summary of radioactive decay processes Caption: Various characteristics of radioactive decay processes. Notes: Keywords: radioactive decay, alpha, beta, gamma, positron, electron capture 2013 Pearson Education, Inc. Chapter 11

Write the equation for the α decay of Th-230: 23090Th → Examples: α Emission Write the equation for the α decay of Th-230: 23090Th → β Emission Write the equation for the loss of a β particle by Pb-210: 21082Pb → 2013 Pearson Education, Inc. Chapter 11

Write the equation for the loss of a positron by Y-85: 8539Y → Examples Cont.: Positron Emission Write the equation for the loss of a positron by Y-85: 8539Y → Electron Capture Write the equation for the electron capture of rubidium-82: 8237Rb + 0-1e → 2013 Pearson Education, Inc. Chapter 11

11.5 Radioactive Half-Life Rates of nuclear decay are measured in units of half-life (t1/2) The amount of time required for one-half of the radioactive sample to decay. Each passage of a half-life causes the decay of one half of whatever sample remains. The half-life is the same no matter what the size of the sample, the temperature, or any other external conditions. Half-life varies from isotope to another Uranium-238 t1/2 = 7.038x108 years Phosphorus-32 t1/2 = 14 days Example of decay: 2013 Pearson Education, Inc. Chapter 11

All nuclear decays follow the same curve. 50% of the sample remains after one half-life 25% after two half-lives 12.5% after three half-lives Etc. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 30

Example of Radioactive Half-Life The half-life of 146C is 5730 years. How much 146C will remain after six half-lives in a sample that initially contained 25.0 g? How long will we need to wait for 6 half-lives? What % of 146C remains in a sample estimated to be 11,000 years old? 2013 Pearson Education, Inc. Chapter 11

Radioisotopes used internally for medical applications have short half-lives. They decay rapidly and do not remain in the body for prolonged periods. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 32

11.6 Radioactive Decay Series Decay series: A series of nuclear disintegrations leading from a heavy radioisotope to a nonradioactive product. Example: Uranium-238 U-238 undergoes a series of 14 sequential nuclear reactions, ultimately stopping at lead-206. Long arrows = α emission Short arrows = β emission 2013 Pearson Education, Inc. Chapter 11 Chapter 11 33

Radon-222 Produced by α decay of radium-226 (half-life = 1600 yrs.). Rocks, soil, building materials that contain radium are sources of radon-222. Radon gas passes in and out of lungs, normally with no effect. If radon-222 decays while in the lungs, polonium-218 is formed polonium-218 can decay emitting α particles. α particles can damage lung tissue. 2013 Pearson Education, Inc. Chapter 11

11.7 Ionizing Radiation A group of high-energy radiation. Includes α particles, β particles, γ rays, X-rays and cosmic rays X-rays are like γ rays (no mass, electromagnetic radiation) but energy is slightly less. cosmic rays = mixture of high-energy particles from outer space (mainly protons with some α and β particles) Molecule → Ion + e- ionizing radiation Ion is very reactive – reacts with other molecules to create fragments. 2013 Pearson Education, Inc. Chapter 11

Can selectively destroy cancer cells. A large dose of ionizing radiation can destroy living cells, causing death. A small dose of ionizing radiation may not cause visible symptoms but might lead to a genetic mutation or cancer. Can selectively destroy cancer cells. 2013 Pearson Education, Inc. Chapter 11

Effect of Ionizing Radiation on the Body Varies with: Energy of radiation Distance from body Length of exposure Location of source (inside or outside of body) If source is outside of body: γ rays and X-Rays more harmful than α and β particles. Greater penetration (Clothing will stop α and β particles). If source is inside of body: α and β particles more harmful than γ rays and X-Rays. Give up energy to surrounding tissue. α emitters especially dangerous (not used in medical applications). 2013 Pearson Education, Inc. Chapter 11

Protection from Radiation Health professionals who work with X rays or other kinds of ionizing radiation protect themselves by surrounding the source with a thick layer of lead or other dense material. Protection is also afforded by controlling the distance between the worker and the radiation source. Radiation intensity (I) decreases with the square of the distance from the source. The intensities (I) of radiation at two different distances (d) are given by the equation: I1d12 = I2d22 Example: A person is exposed to 75 units of radiation if they are standing 1 meter from the radioactive source. What is the exposure if the distance between the person and the source is increase to 2 meters.? 2013 Pearson Education, Inc. Chapter 11 Chapter 11 38

Photographic Film Badge 11.8 Detecting Radiation Photographic Film Badge The simplest device for detecting exposure to radiation. The film is protected from exposure to light, but any other radiation striking the badge causes the film to fog. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 39

Geiger Counter The Geiger counter is an argon-filled tube. The inner walls are given a negative charge, and a wire in the center is given a positive charge. Radiation ionizes the argon atoms, which briefly conduct a current between the walls and the center electrode. The current is detected, amplified, and used to produce a clicking sound or to register on a meter. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 40

Scintillation Counter The most versatile method for measuring radiation in the laboratory is the scintillation counter. In this device, a substance called a phosphor emits a flash of light when struck by radiation. The number of flashes are counted electronically and converted into an electrical signal. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 41

11.9 Measuring Radiation Units of measurement depend on characteristic you are trying to measure. Some radiation intensity units measure the number of nuclear decay events; others measure exposure to radiation or the biological consequences of radiation. Curies and Rems used most often. 2013 Pearson Education, Inc. Chapter 11

Biological Consequences of Radiation The average annual radiation dose is only about 0.27 rem. About 80% of this background radiation comes from natural sources; the remaining 20% comes from medical procedures and from consumer products. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 43

11.10 Artificial Transmutation Very few of the approximately 3300 known radioisotopes occur naturally. Most are made from stable isotopes by artificial transmutation - The change of one atom into another brought about by nuclear bombardment reactions. Atom bombarded with a high-energy particle to create an unstable nucleus. Nuclear change occurs to create a different element. Example: Carbon-14 is created in the upper atmosphere. 147N + 10n  146C + 11H All transuranium elements (elements with atomic numbers greater than 92) have been produced by bombardment reactions. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 44

Example: What isotope is formed by bombardment of nitrogen-14 with alpha particles 147N + 42α → 11H + ? 2013 Pearson Education, Inc. Chapter 11

Body Imaging X-Ray imaging – Most familiar to us. Diagnostic imaging Uses distribution pattern of a radiopharmaceutical. Diseased parts either have a higher or lower concentration than background. Technetium-99m often used (short half-life). CT or CAT Scans Use X-rays PET Scans (Positron Emission Tomography) Combines tomography with radioisotope imaging. Incorporates isotopes into biologically active compounds (glucose). Can be used in cancer detection. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 46

11.11 Nuclear Fission and Nuclear Fusion Nuclear Fission: The fragmenting of heavy nuclei. Uranium-235 is the only naturally occurring isotope that undergoes nuclear fission. Uranium-235 bombarded with neutrons Nucleus splits (>400 different ways with >800 possible products) Frequently occurring pathway of U-235 Fission: 10n + 23592U → 14256Ba + 9136Kr + 3 10n Note: 1 neutron used, 3 neutrons released. Result: chain reaction If a critical mass of U-235 is present, reaction is self-sustaining. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 47

Nuclear Fission Cont. The fission of 235U produces vast amounts of heat that can be used to produce electric power. Ex: 1.0g of U-235 produces 3.4x108 kcal of energy Lithuania and France generate about 86% and 77%, respectively, of their electricity in nuclear plants. About 22% of U.S. electricity is nuclear-generated. Major objections to nuclear power: Safety – explosion not possible but accidental rupture could occur. Waste Disposal – How do we dispose of radioisotopes with long half-lives? 2013 Pearson Education, Inc. Chapter 11 Chapter 11 48

Example of Fission Chain Reaction 2013 Pearson Education, Inc. Chapter 11 Chapter 11 49

Nuclear Fusion Nuclear Fusion: The joining together of light nuclei. Light nuclei such as the isotopes of hydrogen release enormous amounts of energy when they undergo fusion. Fusion reactions of hydrogen nuclei produce helium that powers our sun and other stars. The necessary conditions (very high temperature and pressure) for nuclear fusion are not easily created on Earth. In stars the temperature is on the order of 2 x 107 K and pressures approach 1 x 105 atmospheres. At these extremes, nuclei are stripped of all their electrons and have enough kinetic energy that nuclear fusion readily occurs. Commercial fusion reactors are not yet available. Benefits of nuclear fusion - Inexpensive source of energy with few radioactive by-products Example of nuclear fusion: 21H + 31H → 42He + 10n + energy 2013 Pearson Education, Inc. Chapter 11 Chapter 11 50

Chapter Summary A nuclear reaction changes an atomic nucleus, causing the change of one element into another. A nuclear reaction is balanced when the sum of the nucleons is the same on both sides of the reaction arrow and when the sum of the charges on the nuclei plus any ejected subatomic particles is the same. Radioactivity is the spontaneous emission of radiation from the nucleus of an unstable atom. a radiation consists of helium nuclei, β radiation consists of electrons, and g radiation consists of high-energy light waves. One half-life is the amount of time necessary for one-half of the radioactive sample to decay. High-energy radiation of all types is called ionizing radiation. When ionizing radiation strikes an atom, it makes a reactive ion that can be lethal to living cells. γ rays and X-rays are the most penetrating and most harmful types of external radiation; a and b particles are the most dangerous types of internal radiation. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 51

Chapter Summary The curie (Ci) measures disintegrations per second; the roentgen (R) measures the ionizing ability; the rad measures the amount of radiation energy absorbed per gram of tissue; and the rem measures the amount of tissue damage caused by radiation. Radiation effects become noticeable with a human exposure of 25 rem and become lethal at an exposure above 600 rem. Transmutation is the change of one element into another brought about by a nuclear reaction. Most known radioisotopes do not occur naturally but are made by bombardment of an atom with a high-energy particle. In fission, the nucleus is split apart to give smaller fragments. A large amount of energy is released during fission, leading to its use for generating electric power. Nuclear fusion results when small nuclei such as those of tritium and deuterium combine to give a heavier nucleus. 2013 Pearson Education, Inc. Chapter 11 Chapter 11 52