Chemistry B Unit 6
Who were the first people to experiment with radioactive elements and isotopes?
1896, French chemist Antoine Henri Becquerel was studying the ability of uranium salts that had been exposed to sunlight to fog photographic film plates. During bad weather, when he couldn't expose the salts to sunlight, he left the sample on top of the photographic plate. When de developed the plate, he discovered that the uranium salt had still fogged the film. At the same time, Marie and Pierre Curie (two of Becquerel's associates) were able to show that rays emitted by uranium atoms caused the film to fog. Marie Curie used the term radioactivity to refer to the spontaneous emission of rays or particles from certain elements, such as uranium. The rays and particles emitted from a radioactive source are now called nuclear radiation.
Who was Lyuba?
2007, Russian reindeer herder thought he saw the frozen remains of a reindeer. But he found a perfectly preserved baby wooly mammoth about the size of a large dog. The only thing missing was its long and shaggy coat. Scientists named the mammoth Lyuba (love in Russian).
What is a geiger counter?
A Geiger counter uses a gas-filled metal tube to detect radiation. The tube has a central wire electrode that is connected to a power supply. When ionizing radiation penetrates a thin window at one end of the tube, the gas inside the tube becomes ionized. Because of the ions and free electrons produced, the gas is able to conduct electricity. Each time a Geiger tube is exposed to radiation, current flows. The bursts of current drive electronic counters or cause audible clicks from a built-in speaker. Geiger counters can detect alpha, beta, and gamma radiation. The first small, hand-held Geiger counters were developed in the 1930s. Astronomers use Geiger counters to detect cosmic rays from outer space. Geologists use Geiger counters to search for radioactive minerals, such as uranium ores. These devices are also used to check for leaks in hospitals and other places that use radiation. Figure 25.16 shows one use for a Geiger counter.
What is an atomic bomb?
A device that can trigger an uncontrolled nuclear chain reaction.
What is a gamma ray?
A high energy photon emitted by a radioisotope.
What is a positron?
A particle with the mass of an electron but a positive charge. Its symbol is 0/+1e.
What is neutron moderation?
A process that slows down neutrons to the reactor fuel can capture them to continue the chain reaction. Moderation is necessary because most of the neutrons produced move so fast that they would pass right through a nucleus without being captured. Water and carbon in the form of graphite are good moderators.
What is positron emission?
A proton changes to a neutron, just as in electron capture. For example, 8/5B --> 8/4Be + 0/+1e 15/8O --> 15/7N + 0/+1e When a proton is converted to a neutron, the atomic number decreases by 1 and the number of neutrons increases by 1.
What is scintillation counter?
A scintillation counter uses a phosphor-coated surface to detect radiation. When ionizing radiation strikes the surface, the phosphor produces bright flashes of light, or scintillations. The number of flashes and energies are detected electronically. The data is then converted into electronic pulses, which are measured and recorded. Scintillation counters are more sensitive than Geiger counters. This means that they can detect some radiation that would not be detected by a Geiger counter. Scintillation counters are used to track the path of radioisotopes through the body. They are also used to monitor the possible transport of radioactive materials across national borders and through airports.
How does the conservation of mass theory change in nuclear reactions?
A small amount of mass is converted into energy and is released during radioactive decay, rather than being conserved.
What is nuclear force?
All atomic nuclei, except for hydrogen atoms, consist of neutrons and two or more protons. If a force didn't hold these subatomic particles together, the like-charged protons would repel one another and fly apart. The nuclear force is an attractive force that acts between all nuclear particles that are extremely close together, such as protons and neutrons in a nucleus. At these short distances, the nuclear force dominates over electromagnetic repulsions and holds the nucleus together.
What is alpha emission?
Alpha emission increases the neutron-to-proton ratio, which tends to increase the stability of the nucleus. In alpha emission the mass number decreases by four and the atomic number increases by two. For example, 226/88Ta --> 222/86Rn + 4/2He 232/90Th --> 228/88Ra + 4/2He
What are some practical uses of radioisotopes?
Although radiation can be harmful, it can be used safely and has many important applications. Radioisotopes are used to analyze matter, study plant growth, diagnose medical problems, and treat diseases.
What is a beta particle?
An electron resulting from the breaking of a neutron in an atom.
Comparison of alpha, beta, and gamma particles.
Because of their opposite charges, alpha and beta radiation can be separated by an electric field. In the electric field, alpha, beta and particles pass between two plates with positive and negative charges. The alpha particles swerve up to the negative charge. The beta particles swing down to the positive charge. And gamma rays travel straight because they have no charge and are not attracted to other charges.
What are the half lives of some naturally occurring radioisotopes and what radiation do they emit?
Carbon-14: 5.730 * 10^3 years B Potassium-40: 1.25 * 10^9 years B and y Radon-222: 3.8 days a Radium-226: 1.6 * 10^3 years a and y Thorium-234: 24.1 days B and y Uranium-235: 7.0 * 10^8 years a and y Uranium-238: 4.5 * 10^9 years a
How much of a radioactive sample remains after each half-life?
During each half-life, half of the remaining radioactive atoms decay into atoms of a new element.
What are transuranium elements?
Elements with atomic number above 92, the atomic number of uranium. All of these elements are radioactive. All transuranium elements undergo transmutation. These elements are synthesised in nuclear reactors and nuclear accelerators. Reactors produce beams of low-energy particles. Accelerators are used to increase the speed of bombarding particles to high speeds. sometimes particles must pass through a series of accelerators before they reach the desired speed. CERN has a number of accelerators at its site between the France and Swiss border. When uranium-238 is bombarded with relatively slow neutrons from a nuclear reactor, some uranium nuclei capture these neutrons. The product is uranium-239: 238/92U + 1/0n --> 239/92U Uranium-239 is radioactive and emits a beta particle. The other product is an isotope of the artificial radioactive element neptunium (atomic number 93): 239/92U --> 239/93Np + 0/-1e Neptunium is unstable and decays, emitting a beta particle and a second artificial element, plutonium (atomic number 94): 239/93Np --> 239/94Pu + 0/-1e Plutonium and neptunium are both transuranium elements. The majority of these elements do not occur in nature. Scientists in Berkeley, California, synthesized the first two artificial elements in 1940. Since that time, more than 20 additional transuranium elements have been produced artificially.
What is a half-life?
Every radioisotope has a characteristic rate of decay, which is measured by its half-life. A half-life 9t 1/2) is the time required for one-half of the nuclei in a radioactive sample to decay to products.
What is film badge?
Figure 25.17 is a diagram of a typical film badge. The badge contains layers of photographic film covered with black light-proof paper. The film is sealed in a plastic or metal holder. To reach the film, radiation must pass through a filter, which absorbs some radiation, or a transparent area through which radiation can pass easily. People who work with or near ionizing radiation must wear a film badge to monitor their exposure while they are at work. At specific intervals, the film is removed and developed. The strength and type of radiation exposure are determined by comparing the darkness of the film in all the exposed areas. Records are kept of the results. Film badges do not protect a person from radiation, but they do monitor the degree of exposure. To protect themselves, workers must keep a safe distance from the source and use adequate shielding.
What nuclear waste?
Fuel rods from nuclear power plants are one major source of nuclear waste. The fuel rods are made from a fissionable isotope, either uranium-235 or plutonium-239. The rods are long and narrow—typically 3 meters long with a 0.5-cm diameter. In a typical reactor, three hundred fuel rods are bundled together to form an assembly, and one hundred assemblies are arranged to form the reactor core. During fission, the amount of fissionable isotope in each fuel rod decreases. Eventually the rods no longer have enough fuel to ensure that the output of the power station remains constant. The isotope-depleted, or spent, fuel rods must be removed and replaced with new fuel rods. Spent fuel rods are classified as high-level nuclear waste. They contain a mixture of highly radioactive isotopes, including fission products and what remains of the nuclear fuel. Some of these fission products have very short half-lives, on the order of fractions of seconds. Others have half-lives of hundreds or thousands of years. All nuclear power plants have holding tanks, or "swimming pools," for spent fuel rods. Water cools the spent rods, and also acts as a radiation shield to reduce the radiation levels. The pools, like the one shown in Figure 25.13, are typically 12 meters deep. Storage racks at the bottom of these pools are designed to hold the spent fuel assemblies. The rods continue to produce heat for years after their removal from the core. The spent fuel rods may spend a decade or more in a holding tank. In the past, plant operators expected spent fuel rods to be reprocessed. Any leftover fissionable isotope in the rods would be recycled in the manufacture of new fuel rods. However, with large deposits of uranium ore available—many in the United States—it is less expensive to mine new fuel than to reprocess depleted fuel. At some nuclear plants, the storage pool has no space left. In order to keep these plants open, their fuel rods must be moved to off-site storage facilities. Finding appropriate storage sites is difficult because high-level waste may need be stored for a long time. Plutonium-239, for example, will not decay to safe levels for 240,000 years. Often, people are concerned about having nuclear waste stored nearby or shipped through their communities.
How do fission reactions and fusion reactions differ?
Fusion reactions, in which small nuclei combine, release much more energy than fission reactions, in which large nuclei split apart and form smaller nuclei.
What are three devices used to detect radiation?
Geiger counters, scintillation counters, and film badges are commonly used to detect radiation.
Compare half-lives.
Half-lives can be as short as a second, or as long as a billion years. Scientists use the half-lives of some long-term radioisotopes to determine the age of ancient objects. Many artificially produced radioisotopes have short half-lives, which makes them useful in nuclear medicine. Short-lived isotopes are not a long-term radiation hazard for patients.
What is the decaying process of uranium-238, and what can it be used for?
Having a long half-life, uranium-238 undergoes a complex series of unstable isotopes to the stable isotope of lead-206. The age of uranium-containing minerals can be estimated by measuring the ratio of uranium-238 to lead-206. Because the half-life of uranium-238 is 4.5 * 10^9 years, it is possible to use its half-life to date rocks as old as the solar system.
What is an alpha particle?
Helium nuclei that contains two protons and two neutrons and has a double positive charge.
What happens in a nuclear chain reaction?
In a chain reaction, some of the emitted neutrons react with other fissionable atoms, which emit neutrons that react with still more fissionable atoms.
What are the similarities between chemical and nuclear reactions?
In both reactions, atoms become more stable.
What is a band of stability?
More than 1,500 nuclei are known, and only 264 are stable and do not decay. The rest are unstable and will change over time. The stability of a nucleus depends on the ratio of neutrons to protons. A graph can show the number of neutrons vs. the number of protons for all known stable nuclei. The region of the graph in which these points are located is called the band of stability. For elements of low atomic number (below 20), the ratio is about 1. Above atomic number 20, stable nuclei have more neutrons that protons.
How can radiation be detected?
Radiation emitted by radioisotopes has enough energy to knock electrons off some atoms of a bombarded substance, producing ions. Thus, the radiation emitted by radioisotopes is called ionizing radiation. It is not possible for humans to see, hear, smell, or feel ionizing radiation. So people must rely on detection devices to alert them to the presence of radiation and to monitor its level. These devices work because of the effects of the radiation when it strikes atoms or molecules in the detector. For example, the radiation can expose a photographic plate, which produces an image such as the one shown in Figure 25.15. When the plate is developed, its darkened areas show where the plate has been exposed to radiation. Some devices rely on the current produced when atoms are ionized. Geiger counters, scintillation counters, and film badges are commonly used to detect radiation.
How is radiation used to treat diseases?
Radiation is one method used in the treatment of some cancers. Cancer is a disease in which abnormal cells in the body are produced at a rate far beyond the rate for normal cells. The mass of cancer cells that result from this runaway growth is called a tumor. Fast-growing cancer cells are more susceptible to damage by high-energy radiation such as gamma rays than are healthy cells. Thus, radiation can be used to kill the cancer cells in a tumor. Some normal cells are also killed, however, and cancer cells at the center of the tumor may be resistant to the radiation. Therefore, the benefits of the treatment and the risks to the patient must be carefully evaluated before radiation treatment begins. Cobalt-60 and cesium-137 are typical radiation sources for cancer therapy. Salts of radioisotopes can also be sealed in gold tubes and directly inserted in tumors. This method of treatment is called seeding. The salts emit beta and gamma rays that kill the surrounding cancer cells. Because the radioisotope is in a sealed container, it is prevented from traveling to other parts of the body. Prescribed drugs containing radioisotopes of gold, iodine, or phosphorus are sometimes used in radiation therapy. For example, a dose of iodine-131 larger than that used to detect thyroid diseases can be given to a patient to treat the disease. The radioactive iodine passes through the digestive system into the blood, which carries it to the thyroid. The iodine that collects in the gland emits beta particles and gamma rays, which provide therapy.
How is radiation used as tracers?
Radioisotopes called tracers are used in agriculture to test the effects of herbicides, pesticides, and fertilizers on plants. A tracer is introduced into the substance being tested. Next, plants are treated with the tagged substance. Devices that detect radioactivity are used to locate the substance in the plants. The tracer may also be monitored in animals that consume the plants, as well as in water and soil.
How is radiation used to diagnose medical problems?
Radioisotopes can be used to detect disorders of the thyroid gland, which is located in the throat. The main function of this gland is to control the rate at which your cells release energy from food. The thyroid gland extracts iodide ions from blood and uses them to make the hormone thyroxine. To diagnose thyroid disease, the patient is given a drink containing a small amount of the radioisotope iodine-131. After about two hours, the amount of iodide uptake is measured by scanning the patient's throat with a radiation detector. Figure 25.18 shows the results of such a scan. In a similar way, the radioisotope technetium-99m is used to detect brain tumors and liver disorders. Phosphorus-32 is used to detect skin cancer.
What is radiocarbon dating?
Scientists find the age of an object that was once part of a living system by measuring the amount of carbon-14 (14/6C) it contains. Carbon-14 has a half-life of 5730 years. Most of Earth's carbon, however, consists of more stable isotopes 12/6C and 13/6C. The ratio of 14/6C to the other carbon isotopes in the environment is fairy constant because high-energy cosmic rays from space constantly produce 14/6C in carbon dioxide in the upper atmosphere. Plants use carbon dioxide to produce carbon compounds, such as glucose. In those compounds, the ratio of carbon isotopes is the same as in the air. The same ratio is maintained as animals consume the plants, and other animals. Thus, the ratio of carbon-14 to other carbon isotopes is constant during an organisms lifetime. When an organism dies, it stops exchanging carbon with the environment and its radioactive 14/6C atoms decays without being replaced. Therefor, the ratio fo 14/6C to stable carbon in the remains of an organism changes in a predictable way. Archaeologists can use this data to estimate when an organism died.
How is radiation used to analyse matter?
Scientists use radiation to detect trace amounts of elements in samples. The process is called neutron activation analysis. A sample is bombarded with neutrons from a radioactive source. Some atoms in the sample become radioactive. The half-life and type of radiation emitted can be detected and analyzed by a computer. Because this data is unique for each isotope, scientists can determine what radioisotopes were produced and infer what elements were in the original sample. Museums use this process to detect art forgeries. Crime laboratories use it to analyze gunpowder residues.
Why are some nuclei unstable?
Some nuclei are unstable because they have too many neutrons relative to the number of protons. When one of these nuclei decays, a neutron emits a beta particle (fast-moving electron). A neutron that emits an electron becomes a proton. This is beta emission. 1/0n --> 1/1p + 0/-1e Some nuclei are unstable because they have too few neutrons relative to the number of protons. These nuclei increase their stability by converting a proton to a neutron. An electron is captured by a nucleus during this process. This is called electron capture. All nuclei that have an atomic number greater than 83 have too many neutrons and too many protons to be stable, so they undergo radioactive decay. Most emit alpha particles, which is called alpha emission.
What is alpha radiation?
Some radioactive sources emit helium nuclei, which are called alpha particles. In nuclear equations, an alpha particle is written as 4/2He or a. The electric charge symbol is usually omitted. When an atom loses an alpha particle, the atomic number of the product is lower by two and its mass number is lower by four. In a balanced nuclear equation, the sum of the mass numbers (top one) on the right must equal the sum on the left. The same is true for the atomic numbers (bottom one). For example: Uranium-238 emits alpha radiation and is transformed into another radioisotope, thorium-234. 238/92u --> 234/90Th + 4/2He (a emission) Because of their large mass and charge, alpha particles do not travel very far and are not very penetrating. A sheet of paper or the surface of your skin can stop alpha particles. Radioisotopes that emit alpha particles can cause harm when ingested. Once inside the body, the particles don't have to travel far to penetrate soft tissue.
What are transmutations outside of nature?
Some transmutations that do not occur in nature can be forced to occur in a laboratory or in a nuclear reactor. Ernest Rutherford performed the earliest artificial transmutation in 1919. He bombarded nitrogen gas with alpha particles. As the nitrogen atoms absorb the alpha particles, they form fluorine-18 atoms. 14/7N + 4/2He --> 18/9F The unstable fluorine atoms decay as well: 18/9F --> 17/8O + 1/1p Rutherford's experiment would lead to the discovery of the proton. Rutherford and other scientists noticed a pattern as they did different transmutation experiments. In every case, hydrogen nuclei were emitted. Scientists realised that these hydrogen nuclei (protons) must have a fundamental role in atomic structure. James Chadwick's discovery of the neutron in 1932 also involved a transmutation experiment. Neutrons were produced when beryllium-9 was bombarded with alpha particles: 9/4Be + 4/2He --> 12/6C + 1/0n
What is radioactive decay (type of nuclear reaction)?
The atoms of a radioisotope becomes more stable when changing occur in their nuclei. The changes are always accompanied by the emission of large amount of energy. Radioactive decay is spontaneous, so it does not require an input of energy. If the product of a nuclear reaction is unstable, it will decay as well. The process continues until unstable isotopes of one element are change, or transformed, into stable isotopes of a different element. These stable isotopes are not radioactive. Emits radiation.
What is transmutation?
The conversion of an atom of one element into an atom of another element.
What is gamma radiation?
The high-energy photons, emitted by a radioisotope, are a form of electromagnetic radiation. Called a gamma ray, its symbol is y. Nuclei often emit gamma rays along with alpha or beta particles during radioactive decay. For example: 230/90Th --> 226/88Ra + 4/2He + y 234/90Th --> 234/91Pa + 0/-1e + y Gamma rays have no mass and no electrical charge. So the emission of gamma radiation does not alter the atomic number or mass number of an atom. Because gamma rays are extremely penetrating, they can be very dangerous. Gamma rays can pass through paper, wood, and the human body. They can be stopped, although not completely, by several meters of concrete or several centimeters of lead.
What determines the type of decay a radioisotope undergoes?
The neutron-to-proton ratio in a radioisotope determines the type of decay that it undergoes.
What are transmutations in nature?
The production of carbon-14 from nitrogen-14 that takes place in the upper atmosphere is one example. Recall the decay series of uranium-238. In this series, 14 transmutations occur before a stable isotope of lead is produced.
What is nuclear radiation?
The rays and particles emitted from a radioactive source.
What is radioactivity?
The spontaneous emission of rays or particles from certain elements, such as uranium.
What is fusion?
The sun, directly and indirectly, is the source of most energy used on Earth. The energy emitted by the sun results from nuclear fusion. Fusion occurs when nuclei combine to produce a nucleus of greater mass. In solar fusion, hydrogen nuclei (protons) fuse to make helium nuclei. However, fusion reactions occur only at very high temperatures—in excess of 40,000,000°C.
What are the uses and problems with nuclear fusion?
The use of controlled nuclear fusion as an energy source on Earth is appealing. The potential fuels are inexpensive and readily available. Some scientists are studying a reaction in which a deuterium (hydrogen-2) nucleus and a tritium (hydrogen-3) nucleus combine to form a helium nucleus. 2/1H + 3/1H --> 4/2He + 1/0n + energy The problems with fusion lie in achieving the high temperatures needed to start the reaction and in containing the reaction once it has started. The high temperatures required to start fusion reactions have been achieved by using a fission bomb. Such a bomb is the triggering device used for setting off a hydrogen bomb, which is an uncontrolled-fusion device. This process is of no use, however, as a controlled generator of power.
What are three types of nuclear radiation?
Three types of nuclear radiation are alpha radiation, beta radiation, and gamma radiation.
What is neutron absorption?
To prevent the cahin reaction from going too fast, some of the slowed neutrons must be trapped before they hit fissionable atoms. Neutron absorption is a process that decreases the number of slow-moving neutrons. Control rods, made of materials such as cadmium, are used to absorb neutrons. When the control rods extend almost all the way into the reactor core, they absorb many neutrons, and fission occurs slowly. As the rods are pulled out, they absorb fewer neutrons and the fission process speeds up. If the chain reaction were to go too fast, heat might be produced faster than the coolant could remove it. The reactor core would overheat, which could lead to mechanical failure and release of radioactive materials into the atmosphere. Ultimately, a meltdown of the reactor core might occur.
What are two ways in which transmutation can occur?
Transmutation can occur by radioactive decay or when particles bombard the nucleus of an atom. The particles may be protons, neutrons, alpha particles, or small atoms.
How do nuclear reactions differ from chemical reactions?
Unlike chemical reaction reactions, nuclear reactions are not affected by changes in temperature, pressure, or the presence of catalysts. Also, nuclear reactions of a given radioisotope cannot be slowed down, sped up, or stopped. In a chemical reaction, atoms attain a more stable electron configuration by transferring or sharing electrons. Nuclear reactions begin with radioisotopes.
What are radioisotopes?
Unstable isotopes.
What are two fissionable isotopes?
Uranium-235 and plutonium-239.
What is uranium-235 fission?
Uranium-235 breaks into two smaller fragments roughly the same size when struck by a slow moving neutron. At the same time more neutrons are released by the fission. These neutrons strike the nuclei of other uranium-235 atoms, which causes a chain reaction.
What is beta radiation?
When a neutron breaks apart, it breaks into a proton, which remains in the nucleus, and a fast-moving electron that is released. 1/0n --> 1/1P + 0/-1e The symbol for the electron has a bottom number of -1 and a top number of 0. The -1 represents the charge of the electron. The 0 represents the extremely small mass of the electron compared to the mass of a proton. 14/6C --> 14/7N + 0/-1e (B emission) The mass number is the same, and the atomic number increased by 1. A beta particle has less charge than an alpha particle and much less mass than an alpha particle. Thus, beta particles are more penetrating than alpha particles. Beta particles can pass through paper but are stopped by aluminum foil or thin pieces of wood.
What is beta emission?
When a neutron emits an electron and becomes a proton. It increases the number of protons while decreasing the number of neutrons. For example, 66/29Cu --> 66/30Zn + 0/-1e 14/6C --> 14/7N + 0/-1e
What is electron capture?
When a proton is converted to a neutron, an electron is captured. For example, 59/28Ni + 0/-1e --> 59/27Co 37/18 + 0/-1e --> 37/17Cl
What is fission?
When the nuclei of certain isotopes are bombarded with neutrons, the nuclei split into smaller fragments. This process is called fission. Nuclear fission can release enormous amounts of energy. The fission of 1 kg of uranium-235, for example, yields an amount of energy equal to that produced when 20,000 tons of dynamite explode. In an uncontrolled nuclear chain reaction, all the energy is released in fractions of a second. Fission can be controlled so energy is released more slowly. Nuclear reactors, such as the one shown in Figure 25.12, use controlled fission to produce useful energy. The reaction takes place within uranium-235 or plutonium-239 fuel rods. Much of the energy produced in this reaction is in the form of heat. A fluid, usually liquid sodium or water, removes heat from the core, or central part, of the reactor. Thus, the fluid is called a coolant. The heated fluid is used to change water to steam, which drives a turbine that generates electricity. The control of fission in a nuclear reactor involves two steps, neutron moderation and neutron absorption.
What is an exponential decay function?
You can use the following equation to calculate how much of an isotope will remain after a given number of half lives: A = A(subscript)0 * (1/2)^n In the formula, A stands for the amount remaining, A(subscript)0 for the initial amount, and n for the number of half-lives. The exponent n indicates how many times A(subscript)0 must be multiplied by 1/2 to determine A.
