Radioactive and Nonradioactive Isotopes

Few of the naturally occurring elements have significant amounts of radioactive isotopes, but there are many artificially produced radioactive species. Mass spectrometry can measure both radioactive and nonradioactive isotope ratios, but there are health and safety issues for the radioactive ones. However, modem isotope instmments are becoming so sensitive that only very small amounts of sample are needed. Where radioactive isotopes are a serious issue, the radioactive hazards can be minimized by using special inlet systems and ion pumps in place of rotary pumps for maintaining a vacuum. For example, mass spectrometry is now used in the analysis of Pu/ Pu ratios. 

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This technique is about 1000 times more sensitive than infrared and magnetic resonance spectroscopy, outlined later. It can also be used to detect different masses of radioactive and nonradioactive isotopes of elements. It can distinguish between the carbon 12 and 13 isotopes and also the oxygen 16 and 18 isotopes. By... 

An extremely valuable technique for research that has been developed in recent years is the use of both radioactive and nonradioactive isotopes as. tracers. By the use of these isotopes an element can be observed in the presence of large quantities of the same element. For example, one of the earliest uses of tracers was the experimental determination of the rate at which lead atoms move around through a crystalline sample of the metal lead. This phenomenon is called self-diffusion. If some radioactive lead is placed as a surface layer on a sheet of lead, and the sample is allowed to stand for a while, it can then be cut up into thin sections parallel to the original surface layer, and the radioactivity present in each section can be measured. The presence of radioactivity in layers other than the original surface layer shows that lead atoms from the surface layer have diffused through the metal. 

Matter consists of atoms that are made up of protons (electropositive), electrons (electronegative), and neutrons (electrically neutral). Because the number of electrons and protons is equal, elements, atoms with different numbers of protons, have different numbers of electrons. The chemical properties of an element depend on the number of electrons, but because the electrons have almost no mass, the atomic weight of an element is its number of protons and neutrons. Neutrons are needed to hold the protons together in the nucleus. Isotopes are elements with different numbers of neutrons. Isotopes with too many neutrons are unstable and emit radioactivity. Radioactive and nonradioactive isotopes are used to follow biochemical reactions in health and disease, to date paleontology specimens, usually bones and teeth, and detect traces of life in ancient rocks. 

Naturally occurring and synthetic radioactive and nonradioactive isotopes of the atmospheric gases have been employed for studies on various circulatory and respiratory problems as well as their interrelationship. Probably the most conunonly employed isotope for studying ventilation-perfusion relationships is krypton-85 xenon-133 and oxygen-15 also have been used. Oxygen-15, like the other radioactive gases, can aid in localizing... 

Radioisotopes used as tracers solve scientific mysteries Radioactive and nonradioactive isotopes of the same element act the same way in a chemical reaction. When scientists want to put tags on a compound, they substitute a radioactive isotope for a nonradioactive one in the compound. Then they can use radiation detectors to track and locate the radioisotope tracer. 

All matter is composed of both radioactive and nonradioactive isotopes. Small amounts of radioactive material in the air, water, soil, and so forth make up a part of the background levels. Cosmic rays from outer space continually bombard us with radiation, contributing to the total backgroimd. Owing to the inevitability of background radiation, there can be no such thing as "zero" radiation ... 

Iodine—A nonmetallic solid element there are both radioactive and nonradioactive isotopes of iodine. 

Radioactive isotopes of an element have similar environmental and toxicological behavior to the non-radioactive isotopes. If they are introduced into the body by inhalation ingestion, the chemical properties of the element will determine its ultimate destination in the body. For example, it is well known that strontium and radium are chemically similar to Ca and tend to deposit on bone surfaces and bone marrow with that element tritium ( H) becomes incorporated into water molecules and is distributed throughout the body. The main difference between radioactive and nonradioactive isotopes is that the radionuclides emit one or more types of radiation as they decay to more stable forms. The type and energy level of the emitted radiation is nuclide-specific. 

Isotope Dilution Another important quantitative radiochemical method is isotope dilution. In this method of analysis a sample of analyte, called a tracer, is prepared in a radioactive form with a known activity. Ax, for its radioactive decay. A measured mass of the tracer, Wf, is added to a sample containing an unknown mass, w, of a nonradioactive analyte, and the material is homogenized. The sample is then processed to isolate wa grams of purified analyte, containing both radioactive and nonradioactive materials. The activity of the isolated sample, A, is measured. If all the analyte, both radioactive and nonradioactive, is recovered, then A and Ax will be equal. Normally, some of the analyte is lost during isolation and purification. In this case A is less than Ax, and... 

In some cases, isotope enrichment, particularly of stable and nonradioactive isotopes, is used to follow a particular element through the environment or through biological systems. The advantage of stable isotopes is that they are not radioactive and thus are easier to handle and dispose. 

A few natural isotopes are radioactive. Of the three isotopes of hydrogen, only that of mass 3 (tritium) i.s radioactive. Radioactive isotopes can be examined by other instrumental means than mass spectrometry, but these other means cannot see the nonradioactive isotopes and are not as versatile as a mass Spectrometer. 

C22-0124. A small amount of NaBr containing the radioactive isotope sodium-24 is dissolved in a hot solution of sodium nitrate containing the naturally occurring nonradioactive isotope sodium-23. The solution is cooled, and sodium nitrate precipitates from the solution. Will the precipitate be radioactive Explain your answer. 

Thulium is relatively scarce and expensive, which hmits its commercial uses. Thulium-170, which is a radioactive isotope of thuhum produced by fission in nuclear reactors, can be used as small, portable X-ray sources. It also has limited use as an alloy metal with other metals and has experimentally been used in lasers. (Note Of all the isotopes of thuhum, only thuhum-169 is stable and nonradioactive.)... 

Why do some nuclei undergo radioactive decay while others do not Why, for instance, does a carbon-24 nucleus, with six protons and eight neutrons, spontaneously emit a /3 particle, whereas a carbon-23 nucleus, with six protons and seven neutrons, is stable indefinitely Before answering these questions, it s important to define what we mean by "stable." In the context of nuclear chemistry, we ll use the word stable to refer to isotopes whose half-lives can be measured, even if that half-life is only a fraction of a second. We ll call those isotopes that decay too rapidly for their half-lives to be measured unstable, and those isotopes that do not undergo radioactive decay nonradioactive, or stable indefinitely. 

Radioactive isotopes have the same chemical properties as the nonradioactive isotopes of the same element. Because they undergo the same chemical reactions, radioactive atoms are often used as tracers to determine what ordinary atoms are doing. For example, to detect problems in the human thyroid gland, physicians often presCTibe iodine that includes a tiny fraction of 53 , a radioactive isotope of iodine. The body should utilize all the iodine in the thyroid gland. With a Geiger counter, the physician can follow the path of the radioactive isotope. If the radioactive iodine is not absorbed by the thyroid, then the regular iodine has not been absorbed either, and the physician has confirmed that a certain problem exists. 

Radioactive isotopes react chemically just like the nonradioactive isotopes of the same element. Because of this, physicians can add a little radioactive isotope to a sample of an element, which is then ingested or injected into the body. They can then determine the element s location in the body by detecting the particles that the radioactive isotope emits. The radioactive isotope is called a tracer, and this technique is used extensively in medicine and other fields (Section 21.1). 

If a radioisotope is substituted for a nonradioactive isotope of the same element in a chemical reaction, aU compounds formed from that element in a series of steps will also be radioactive. That makes it possible to follow the reaction pathway using instruments that can detect radiation. In this way, the series of steps involved in many important reactions has been studied, as shown in Figure 21.17. Tracers containing radioactive phosphorus-32 have also been used in biochemical research to help clarify complicated metabolic pathways. Tracers are also used to test structural weaknesses in mechanical equipment and to follow the pathways taken by pollutants. 

PROBLEM 6.14 The mass 82 isotope of bromine ( Br) is radioactive and is used as a tracer to identify the origin and destination of individual atoms in chemical reactions and biological transformations. A sample of 1,1,2-tribromocyclohexane was prepared by adding Br— Br to ordinary (nonradioactive) 1-bromocyclo-hexene. How many of the bromine atoms in the 1,1,2-tribromocyclohexane produced are radioactive Which ones are they ... 

Certain isotopes (radioactive isotopes) of elements emit particles and energy that can be used to trace the behavior of biochemical systems. These isotopes otherwise behave identically to any other isotope of the same element. Their chemical behavior is identical it is their nuclear behavior that is unique. As a result, a radioactive isotope can be substituted for the "nonradioactive" isotope, and its biochemical activity can be followed by monitoring the particles or energy emitted by the isotope as it passes through the body. 

The radioactive isotope of an element chosen for tracer studies has exactly the same chemical behavior as any other isotope of the same element. For example, iodine-127, the most abundant nonradioactive isotope of iodine, tends to concentrate in the th)rroid gland. Both radioactive iodine-131 and iodine-125 behave in the same way and are used to study the th)rroid. The rate of uptake of the radioactive isotope gives valuable information regarding underactivity or overactivity (hy-poactive or h)rperactive th)rroid). 

In the use of radioactive tracers it is assumed that the radioactive isotopes studied are identical in chemical behavior to the nonradioactive isotopes. The first experiments that used radioactive tracers were carried out in 1913 in Germany and were designed to measure the solubility of lead salts via the use of a radioactive isotope of lead. In industry, radionuclides have been used for analytical purposes, for measurements of flow in pipes, and as part of many other apphcations. Another example of an important tracer study has been the investigation of photosynthesis of carbohydrates from atmospheric CO2 in the presence of light and chlorophyll. Scientists used eC, 15P, and iH to identify the intermediate steps involved in the photosynthesis of carbohydrates in plants that had been placed in an atmosphere composed of fyC-labeled CO2 and had been irradiated with hght. The presence of the radioactive carbon in the synthesized carbohydrate was evidence that O2 was involved in the synthesis. 

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