Chemical elements isotopes

TABLE 21.1 Definitions of Atoms, Chemical Elements, Isotopes, Nuclides, and Isomers... 

The models and material property data for predicting fission gas release from heavy metal contamination and failed particles are described in Refs. 2 and 3. These models give the release-rate-to-birthrate ratio (R/B) from contamination and failed particles as a function of chemical element, isotope half-life, temperature, and burnup. In addition, the effect of fuel... 

Nowadays, chemical elements are represented in abbreviated form [2]. Each element has its ovm symbol, which typically consists of the initial upper-case letter of the scientific name and, in most cases, is followed by an additional characteristic lower-case letter. Together with the chemical symbol, additional information can be included such as the total number of protons and neutrons in the nucleus, the atomic number (the number of protons in the nucleus) thus isotopes can be distinguished, e.g., The charge value and, finally, the number of atoms which are present in the molecule can be given (Figure 2-3). For example, dioxygen is represented by O2. 

Fermi had been fascinated by the discovery of the neutron by James Chadwick in 1932. He gradually switched his research interests to the use of neutrons to produce new types of nuclear reactions, in the hope of discovering new chemical elements or new isotopes of known elements. He had seen at once that the uncharged neutron would not be repelled by the positively-charged atomic nucleus. For that reason the uncharged neutron could penetrate much closer to a nucleus without the need for high-energy particle accelerators. lie discovered that slow neutrons could... 

Most of the chemical elements consist of mixtures of isotopes. Oxygen, atomic number 8, has three stable isotopes. The kind having mass number 16 is most abundant. About 99.76% of the oxygen atoms consist of this isotope. Only 0.04%... 

But what would become of Mendeleev s periodic system which now seemed to consist of 300 or so "elements" To some chemists, the discovery of isotopes implied the end of the periodic system as it was known.3 These chemists suggested that it would be necessary to consider the individual new isotopes as the new "elements." But the chemist Paneth adopted a less reductionist approach, arguing that the periodic table of the familiar chemical elements should be retained because it dealt with the "elements" that were of interest to chemists. A justification for this view was provided by the fact that, with a few exceptions, the chemical properties of isotopes of the same element are indistinguishable.4 Moreover, Paneth appealed to Mendeleev s distinction between the two senses of the concept of an "element" in order to provide a philosophical rationale for the retention of the chemist s periodic table. Paneth argued that the discovery of isotopes of the elements represents the discovery of new elements as simple substances, whereas periodic... 

An isotope is usually specified by its mass number. The mass number is the total number of protons and neutrons contained in a nucleus. We can represent any isotope of a chemical element completely by writing its chemical symbol (AT) preceded by a superscript giving its mass number A) and a subscript giving its atomic number (Z) Mass number A... 

Hydrogen, the simplest of all chemical elements, has two naturally occurring isotopes. Its most common isotope,... 

A sample of a chemical element is usually a mixture of all of its stable isotopes. The carbon in a sample of 12 13... 

The defining characteristic of an atom of a chemical element is the number of protons in its nucleus. A given element may have different isotopes, which are nuclei with the same numbers of protons but different numbers of neutrons. For example, 12C and 14C are two isotopes of carbon. The nuclei of both isotopes contain six protons. However, 12C has six neutrons, whereas 14C has eight neutrons. In general, it is the number of protons and electrons that determines chemical properties of an element. Thus, the different isotopes of an element are usually chemically indistinguishable. These isotopes, however, have different masses. 

Chemical elements including technetium are being produced in nuclear reactions occurring in the stars today. This has been proved by observing of the presence of technetium in some stars [1]. Technetium has no stable isotopes and none of the technetium isotopes has a half-life long enough to survive the age of the universe. So the technetium observed must have been synthesized by nuclear processes in the stars. 

Q Categorial variable characterizing chemical entities (species) being investigated, e.g., elements, isotopes, ions, compounds ... 

There are at present 116 known chemical elements. However, there are well over 2000 known nuclear species as a result of several isotopes being known for each element. About three-fourths of the nuclear species are unstable and undergo radioactive decay. Protons and neutrons are the particles which are found in the nucleus. For many purposes, it is desirable to describe the total number of nuclear particles without regard to whether they are protons or neutrons. The term nucleon is used to denote both of these types of nuclear particles. In general, the radii of nuclides increase as the mass number increases with the usual relationship being expressed as... 

The existence and distribution of the chemical elements and their isotopes is a consequence of nuclear processes that have taken place in the past in the Big Bang and subsequently in stars and in the interstellar medium (ISM) where they are still ongoing. These processes are studied theoretically, experimentally and obser-vationally. Theories of cosmology, stellar evolution and interstellar processes are involved, as are laboratory investigations of nuclear and particle physics, cosmo-chemical studies of elemental and isotopic abundances in the Earth and meteorites and astronomical observations of the physical nature and chemical composition of stars, galaxies and the interstellar medium. 

Nuclei of natural isotopes (atoms of chemical elements differing in the number of neutrons in their nuclei) may possess angular momentum or spin and therefore magnetic moments. One defines spin by the following equation ... 

Grignon N, Halpem S, Jeusset J, Briancon C, Fragu P. Localization of chemical elements and isotopes in the leaf of soybean (Glycine max) by secondary ion mass spectrometry critical choice of sample preparation procedure. JMicrosc 1997 186 51-66. 

As was mentioned above, accurate mass measurements may establish exact elemental composition of ions. However, as the majority of chemical elements exist as a mixture of several isotopes, the elemental composition may be established more or less reliably using low resolution mass spectra. The presence of isotopes may be demonstrated by spectra of carbon disulfide (Fig. 5.19a), chloroethane (Fig. 5.19b) and bromoethane (Fig. 5.19c). 

TABLE 5.4. Natural Isotopic Abundances of Widespread Chemical Elements... 

Isotope Any of two or more species of atoms of a chemical element with the same atomic number and different atomic mass. For example, U-238 and U-235 are both isotopes of uranium. 

Hydrogen The first chemical element in the periodic table. It has the atomic symbol H, atomic number 1, and atomic weight 1. It exists, under normal conditions, as a colorless, odorless, tasteless, diatomic gas. Hydrogen ions are protons. Besides the common HI isotope, hydrogen exists as the stable isotope deuterium and the unstable, radioactive isotope tritium. [NIH]... 

Bigeleisen, J. Nuclear size and shape effects in chemical reactions. Isotope chemistry of the heavy elements, J. Am. Chem. Soc., 118, 3676 (1996). 

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