chapter eleven nuclear chemistry fundamentals of general, organic and biological chemistry 6th...

Chapter ElevenNuclear Chemistry

Fundamentals of General, Organic and Biological

Chemistry

6th Edition

James E Mayhugh

Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Chapter Eleven 2

Outline

► 11.1 Nuclear Reactions► 11.2 The Discovery and Nature of Radioactivity► 11.3 Stable and Unstable Isotopes► 11.4 Nuclear Decay► 11.5 Radioactive Half-Life► 11.6 Radioactive Decay Series► 11.7 Ionizing Radiation► 11.8 Detecting Radiation► 11.9 Measuring Radiation► 11.10 Artificial Transmutation► 11.11 Nuclear Fission and Nuclear Fusion


Goals

►1. What is a nuclear reaction, and how are equations for nuclear reactions balanced? Be able to write and balance equations for nuclear reactions.

►2. What are the different kinds of radioactivity? Be able to list the characteristics of three common kinds of radiation— , , and (alpha, beta, and gamma).

►3. How are the rates of nuclear reactions expressed? Be able to explain half-life and calculate the quantity of a radioisotope remaining after a given number of half-lives.


Goals Contd.

►4. 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.

►5. How is radioactivity measured? Be able to describe the common units for measuring radiation.

►6. What is transmutation? Be able to explain nuclear bombardment and balance equations for nuclear bombardment reactions.

►7. What are nuclear fission and nuclear fusion? Be able to explain nuclear fission and nuclear fusion.


11.1 Nuclear Reactions► The atomic number, written below and to the left of

the element symbol, gives the number of protons in the nucleus and identifies the element.

► The mass number, written above and to the left of the element symbol, gives the total number of nucleons, a general term for both protons (p) and neutrons (n).

► The most common isotope of carbon, for example, has 12 nucleons: 6 protons and 6 neutrons:


► 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.

► The energy change accompanying a nuclear reaction can be up to several million times greater than that accompanying a chemical reaction.


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 and beta. Shortly thereafter, a third type of radiation was found and named for the third Greek letter, gamma.


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 (), do not bend toward either

plate because they have no charge.


►Alpha rays move about ~0.1c and can be stopped by a few sheets of paper or by the top layer of skin.

►Beta rays move at up to 0.9c and have about 100 times the penetrating power of particles. A block of wood or heavy clothing is necessary to stop rays.

►Gamma rays move at c and have about 1000 times the penetrating power of rays. A lead block several inches thick is needed to stop rays.


11.3 Stable and Unstable Isotopes►Every element in the periodic table has at least one

radioactive isotope, or radioisotope, and more than 3300 radioisotopes are known.

►Their radioactivity is the result of having unstable nuclei, although the exact causes of this instability are not fully understood. 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.


►For elements in the first few rows of the periodic table, stability is associated with a roughly equal number of neutrons and protons.

►As elements get heavier, the number of neutrons relative to protons in stable nuclei increases.

►Lead-208, for example, the most abundant stable isotope of lead, has 126 neutrons and 82 protons in its nuclei.


11.4 Nuclear Decay

► Nuclear decay: The spontaneous emission of a particle from an unstable nucleus.

► Transmutation: The change of one element into another.

► The equation for a nuclear reaction is not balanced in the usual chemical sense because the kinds of atoms are not the same on both sides of the arrow. A nuclear equation is balanced when the number of nucleons and the sums of the charges are the same on both sides.


► During alpha emission, the nucleus loses two protons and two neutrons.

► Emission of an particle from an atom of uranium-238 produces an atom of thorium-234.


► Beta emission involves the decomposition of a neutron to yield an electron and a proton.

► Iodine-131, a radioisotope used in detecting thyroid problems, undergoes nuclear decay by emission to yield xenon-131.


► 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.

► Potassium-40 undergoes positron emission to yield argon-40.


► 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.

► The conversion of mercury- 197 into gold-197 is an example of electron capture.


► Emission of rays causes no change in mass or atomic number.

► emission usually accompanies emission of other rays but it is often omitted from nuclear equations.

► Their penetrating power makes them both dangerous to humans and useful in medical applications.


11.5 Radioactive Half-Life

► Rates of nuclear decay are measured in units of half-life , defined as 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.


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, and so on.


Radioisotopes used internally for medical applications have short half-lives so that they decay rapidly and do not remain in the body for prolonged periods.


11.6 Radioactive Decay Series►Decay series: A

series of nuclear disintegrations leading from a heavy radioisotope to a nonradioactive product.

►Uranium-238, for example, undergoes a series of 14 sequential nuclear reactions, ultimately stopping at lead-206.


11.7 Ionizing Radiation► 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.


► 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 because 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 = I2d2

2


11.8 Detecting Radiation

►The simplest device for detecting exposure to radiation is the photographic film badge.

►The film is protected from exposure to light, but any other radiation striking the badge causes the film to fog.

►We cannot see, hear, smell, touch, or taste radiation, no matter how high the dose. We can, however, detect radiation by taking advantage of its ionizing properties.


►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.


► 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.


11.9 Measuring RadiationSome Radiation intensity units measure the number of nuclear decay events; others measure exposure to radiation or the biological consequences of radiation.


► The biological consequences of different radiation doses are shown below.

► 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.


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.

► Carbon -14 is created in the upper atmosphere.14N + 1n 14C + 1H

► Plutonium-239 is made in breeder reactors.238U + 1n 239U 239Np + - 239Pu + -


11.11 Nuclear Fission and Nuclear Fusion

► Nuclear fission: The fragmenting of heavy nuclei.► Chain reaction: A reaction that is self-sustaining.► Critical mass: The minimum amount of radioactive

material needed to sustain a nuclear chain reaction.► The fission of 235U produces vast amounts of heat that

can be used to produce electric power. ► Lithuania and France generate about 86% and 77%,

respectively, of their electricity in nuclear plants. About 22% of U.S. electricity is nuclear-generated.


Each fission event produces additional neutrons that induce more fissions. Such chain reactions usually lead to the formation of many different fission products.


► 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. It is fusion reactions of hydrogen nuclei to produce helium that powers our sun and other stars.

► The necessary conditions 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.


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. radiation consists of helium nuclei, radiation consists of electrons and 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.


Chapter Summary Contd.► 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.

► rays and X rays are the most penetrating and most harmful types of external radiation; and particles are the most dangerous types of internal radiation.

► 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.


Chapter Summary Contd.

►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.


Key Words

►Alpha () particle►Artificial transmutation►Beta () particle►Chain reaction►Cosmic rays►Critical mass

►Decay series►Electron capture►Gamma () radiation►Half-life►Ionizing radiation►Nuclear decay


Key Words Contd.

►Nuclear fission►Nuclear fusion►Nuclear reaction►Nucleon►Nuclide►Positron

►Radioactivity►Radioisotope►Radionuclide►Transmutation►X rays


End of Chapter 11

chapter eleven nuclear chemistry fundamentals of general, organic and biological chemistry 6th...

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