Nuclear Chemistry - chemistry.

Nuclear Chemistry - chemistry. I INTRODUCTION Nuclear Chemistry, the study of atomic nuclei, especially of radioactive nuclei, and their reactions with neutrons and other nuclei (see Atom). II NUCLIDE DECAY Atomic nuclei consist of positively charged protons and neutral, or uncharged, neutrons (see Neutron; Proton). The number of protons in a nucleus is also the atomic number, which defines the chemical element. All nuclei with 11 protons for example, are nuclei of sodium (Na) atoms (see Elements, Chemical). An element can have various isotopes the nuclei of which have differing numbers of neutrons (see Isotope). For example, stable sodium nuclei contain 12 neutrons, whereas those with 13 are radioactive. These isotopes are notated as ®Na12 and ² Na13, where the left-hand subscript indicates the atomic number and the right-hand, the number of neutrons. The superscript represents the total number of nucleons, or neutrons and protons. Any species of nucleus designated by certain atomic and neutron numbers is called a nuclide. Radioactive nuclides are unstable: They undergo spontaneous transformation into nuclides of other elements, releasing energy in the process (see Radioactivity). These transformations include alpha (?) decay (the emission of a helium nucleus, ¸ He«+ ), and beta (? ) decay or positron (?+ ) decay. In ? decay a neutron is transformed into a proton with the simultaneous emission of a high-energy electron. In ?+ decay a nuclear proton converts into a neutron with the emission of a high-energy positron (see Elementary Particles). For example, 24Na undergoes ? decay to form the next higher element, magnesium: ² Na13-> ³ Mg 12 + ? + g rays Gamma (g) radiation, like light, is electromagnetic radiation, but by virtue of their much higher frequency, g rays are enormously more powerful. When ? or ? decay occurs, the resulting nucleus is often left in an excited (higher energy) state. Gamma rays are emitted as the nucleus drops to a lower energy state. See also Electron; Positron. Any characterization of radioactive nuclide decay must include a determination of the half-life of the nuclide, that is, the time it takes for half of a sample to decay. The half-life of III 24Na, for example, is 15 hours. Nuclear chemists also determine the types and energies of radiation emitted by the nuclide. EARLY EXPERIMENTS Radioactivity was discovered in uranium salts by the French physicist Henri Becquerel in 1896. In 1898 the French scientists Marie and Pierre Curie discovered the naturally occurring radioactive elements polonium ( 84Po) and radium (88Ra). During the 1930s, Irène and Frédérick Joliot-Curie made the first artificial radioactive nuclides by bombarding boron (5B) and aluminum ( 13Al) with ? particles to form radioactive isotopes of nitrogen ( 7N) and phosphorus (15P). Naturally occurring isotopes of these elements are stable. The German nuclear chemists Otto Hahn and Fritz Strassmann discovered nuclear fission in 1938. When uranium is irradiated with neutrons, some uranium nuclei split into two nuclei of about half the atomic number of uranium. Fission releases enormous energy and is used in nuclear fission weapons and reactors (see Nuclear Energy). IV NUCLEAR REACTIONS Nuclear chemistry also involves the study of nuclear reactions: the use of nuclear projectiles to convert one species of nucleus into another. If, for example, sodium is bombarded with neutrons, some of the stable ®Na12 nuclei capture neutrons to form radioactive ² Na13 nuclei: ®Na12 + ¦ n 1-> ² Na13 + g rays Neutron reactions are studied by placing samples inside nuclear reactors, which produce a high neutron flux (high number of neutrons per unit area). Nuclei can also react with each other, but being positively charged, they repel each other with great force. The projectile nucleus must have a high energy to overcome the repulsion and to react with target nuclei. High-energy nuclei are produced in cyclotrons, Van de Graaff generators, or other electronuclear accelerators. See Particle Accelerators. A typical nuclear reaction is the one that was used to produce artificially the next heavier element above uranium (° U), the heaviest element that occurs in nature (see Periodic Law). Neptunium (° Np) was made by bombarding uranium (mostly ° U) with deuterons (nuclei of the heavy hydrogen isotope, ª H1) to knock out two neutrons, forming ± Np: ° U 146 + ª H1-> ± Np145 + 2¦ n 1 V RADIOCHEMICAL ANALYSIS Alpha particles, most of which are emitted by elements with atomic numbers above 83, have discrete energies characteristic of the emitting nuclide. Thus, ? emitters can be identified by measuring the energies of the ? particles. The samples being measured must be very thin, as ? particles lose energy rapidly on passing through material. Gamma rays also have discrete energies characteristic of the decaying nuclide, so g-ray energies can also be used to identify nuclides. Because g rays can pass through considerable material without losing energy, samples need not be thin. Beta-particle (and positron) energy spectra are not useful for identifying nuclides; they are spread over all energies up to a maximum for each ? emitter. See Particle Detectors. Nuclear-chemical techniques are frequently used to analyze materials for trace elements--elements that occur in minute amounts. The technique used is called activation analysis. A sample is irradiated with nuclear projectiles, usually neutrons, to convert stable nuclides into radioactive ones, which are then measured with nuclear radiation detectors. For example, any sodium in a sample can be detected by irradiating the sample with neutrons, thereby converting some of the stable ®Na12 nuclei into radioactive 24Na and measuring the amount of 24Na by counting the ? particles and u rays emitted. Activation analysis can (without chemical separation) measure nanogram (4 × 10-11 oz) concentrations of about 35 elements in such materials as soil, rocks, meteorites, and lunar samples. Activation analysis can be used to analyze biological samples, such as human blood and tissue; however, fewer elements can be observed in biological materials without chemical separations. Other important applications of nuclear chemistry include the development of methods for the production of radioactive species used for medical diagnoses and treatments, as well as for radioactive isotopic tracers (see Isotopic Tracer), which are used in studies of the chemical behavior of elements and are used to measure wear in automobile engines, and in other studies involving extremely small amounts of material. See also articles on the chemical elements mentioned above. Contributed By: Glen E. Gordon Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.