Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Section 18.3 Notes—Uses of Nuclear Chemistry Objectives: 1. Define the half-life of a radioactive nuclide, and explain how it can be used to determine an object’s age. 2. Describe some of the uses of nuclear chemistry. 3. Compare acute and chronic exposures to radiation. Half-life Half-life is the time required for half of a sample of radioactive substance to disintegrate by radioactive decay or natural processes. A radioactive substance decays at a constant rate that is unique to the particular substance. The rate of decay is not influenced by external conditions, such as temperature or pressure. Radioactive Isotope Half-life Polonium-215 0.0018 seconds Bismuth-212 60.5 seconds Sodium-24 15 hours Iodine-131 8.07 days Cobalt-60 5.26 years Radium-226 1600 years Uranium-238 4.5 billion years Radioactive Dating The fact that radioactive decay is constant and is not influenced by external conditions makes it possible to determine the age of an object based on the amount of radioactive isotope that it contains. An example of radioactive dating involves the radioactive isotope carbon-14. Nearly all of the carbon on earth is present as the stable carbon-12 isotope. However, there is also a small percentage is the unstable carbon-14 that decays to form nitrogen-14. The ratio between carbon-12 and carbon-14 in the environment remains constant through time. Both carbon-12 and carbon-14 have the same electronic configuration, so they undergo the same chemical reactions (have the same chemical properties. Both isotopes are in carbon dioxide, which is used by plants in photosynthesis. As a result, all plants and animals that eat plants contain the same ratio of carbon-14 to carbon-12. Any carbon-14 that decays while the organism is alive is replaced through photosynthesis or eating. But when a plant or animal dies, it stops taking in carbon-containing substances, so the carbon-14 that decays is not replaced and the amount decreases over time. The half-life of carbon-14 is 5715 years. By analyzing organic material left from dead plants and animals, scientists can estimate the age of an object by comparing the amount of carbon-14 present to the amount in a living organism. Carbon-14 is useful for dating specimens less than 50,000 years old (or about nine half-lives). Scientists can date ancient rocks on the order of geologic time using Potassium-40, which has a half-life of 1.28 billion years. Other Uses of Nuclear Chemistry Smoke Detectors Today, nuclear reactions have become part of modern living. Smoke detectors contain a small amount of americium-241, which decays to form neptunium-237 and alpha particles: 241 95𝐴𝑚 → 237 93𝑁𝑝 + 42𝐻𝑒 The alarm is set off when the alpha particles contact the smoke particle. Art Forgeries A process called neutron activation analysis to determine whether artwork is a forgery. In this process, a tiny sample of the artwork is bombarded by neutrons. After some of the atoms in the sample absorb neutrons, they become radioactive isotopes and emit gamma rays as they decay. Scientists can then identify each element in the sample by the characteristic gamma ray pattern that the sample’s atoms emit. This creates a “fingerprint” of the elements in the sample. If the fingerprint matches materials not available when the artwork was supposedly created, then the artwork is a fake. Nuclear Medicine Nuclear medicine includes the use of nuclear reactions both to diagnose certain conditions and to treat a variety of diseases. There are many uses of radioactive isotopes in medicine, such as: Doctors can create images of bones using x-rays. But x-rays cannot produce images of softer tissues such as muscle. Doctors can take a close look at a person’s heart using a thallium stress test. The person is given an intravenous injection of thallium-201, which acts chemically like calcium and collects in the heart muscle. As the thallium-201 decays, low-energy gamma rays are emitted and are detected by a special camera that produces images of the heart muscle. The radioactive isotope technetium-99 emits low-energy gamma rays. Bones that are repairing themselves absorb minerals at a certain rate and take up technetium at the same time. Because of this, doctors can monitor diseases that cause fracture, infection, arthritis, or an invading cancer. Doctors use a process called positron emission tomography (PET) to create detailed three-dimensional pictures of a person’s organs. In this procedure, the patient is injected with a radioactive isotope that that has a short half-life and ejects positrons. As the positrons collide with electrons, gamma rays are produced, which are detected by a scanner. Radiation Exposure to the Sun Exposure to radiation can affect a person’s health. The affect on humans is measured using a unit called a rem. A rem (Roentgen Equivalent Man) is a unit used to measure a dosage of ionizing radiation. Effect of Whole-Body Exposure to a Single Dose of Radiation Dose (rem) probable effect 0-25 no observable effect 25-50 slight decrease in white blood cell count 50-100 marked decrease in white blood cell count 100-200 nausea, loss of hair 200-500 ulcers, internal bleeding > 500 death Exposure to a single large dose of radiation are said to have experienced an acute radiation exposure. More than 230 people suffered acute radiation sickness and 28 died when a meltdown occurred in 1986 at the Chernobyl nuclear power plant in the Ukraine. The effects of nuclear radiation on the body can add up over time. Exposure to small doses of radiation over a long period can be as dangerous as a single large dose. Some scientific studies have shown a correlation between chronic radiation exposure and certain types of cancer.