Isotopes and nuclides

For the present purposes the nucleus of an atom can be considered to be made up of protons and neutrons, collectively known as nucleons. A proton has a charge of +1 and the number of protons in a nucleus determines the proton number (also called the atomic number), Z, of the atom. The neutron bears no charge but has a similar mass to that of the proton. The number of electrons in a neutral atom is also equal to Z. Atoms with the same value of Z are chemically identical. The total number of nucleons in the nucleus is called the nucleon number (also called the mass number), A. The number of neutrons in a nucleus need not be the same as the number of protons. Atoms with the same value of Z but different numbers of neutrons are known as isotopes of the element in question. Isotopes are represented by the symbol X. For example, the radioactive 

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H, C, P, and l. The word isotope comes from Greek, meaning at the same place , a useful way to remember that all isotopes of an element are in the same place in the Periodic Table of elements. While not quite correct, often the words isotope and nuclide are used interchangeably. Thus, radioisotopes may be termed radionuclides. The later refers to an atom with an unstable nucleus that undergoes radioactive decay, and these may be naturally occurring or artificially produced. 

In graphs like Fig. 3.1, Z is conunonly plotted as the abscissa we have here reversed the axes to conform with the commercially available isotope and nuclide charts. 

Elements with radioactive nuclides amongst their naturally occurring isotopes have a built-in time variation of the relative concentration of their isotopes and hence a continually... 

Each particular type of nucleus is called a nuclide. Nuclides are characterized by the number of protons (Z) and neutrons (N) that they possess. The number of protons in a nuclide is always the same as the atomic number of the element, Z. Recall from Chapter 2, however, that the number of neutrons can vaiy, and that isotopes are nuclides... 

Krishnaswami S, Lai D, Somayajulu BLK (1976) Large-volume in-situ filtration of deep Pacific waters Mineralogical and radioisotope studies. Earth Planet Sci Lett 32 420-429 Krishnaswami S, Mangini A, Thomas JH, Sharma P, Cochran JK, Turekian KK, Parker PD (1982) °Be and Th isotopes in manganese nodules and adjacent sediments nodule growth histories and nuclide behavior. Earth Planet Sci Lett 59 217-234... 

Th and 231Pa are ubiquitous components of recently deposited deep-sea sediments because they are produced uniformly throughout the ocean from the decay of dissolved uranium isotopes and they are actively collected onto sinking particles. The distribution with depth of these nuclides in deep-sea sediments may be modeled to estimate rates of sedimentation extending over the past 200 to 300 thousand years. These techniques complement 14C dating methods that only extend to about 40 thousand years before the present. 

Modifier D is used to show the mass number of the atom being considered, this being the total number of neutrons and protons considered to be present in the nucleus. The number of protons defines the element, but the number of neutrons in atoms of a given element may vary. Any atomic species defined by specific values of atomic number and mass number is termed a nuclide. Atoms of the same element but with difierent atomic masses are termed isotopes, and the mass number can be used to designate specific isotopes. 

Once the primary isotopes had built up in abundance, then the full range of reactions discussed above became available and nuclides such as C, N, O, O, F, Ne, Ne, 25Mg, 26Mg, and so on, were produced by reactions between primary isotopes and protons and neutrons. Isotopes that require the presence of metals in the initial composition of the star in order to be efficiently synthesized are secondary isotopes. Abundances of primary isotopes built up rapidly in the early universe via synthesis in massive stars. Secondary isotopes initially built up more slowly, but their rate of synthesis increased as metallicity increased. 

A nuclide may be defined as a species of atoms, wilh specified atomic number and mass number. The term nuclide should be used, not isotope. Different nuclides having the same atomic number wv isotopes. Different nuclides having the same mass number are isobars. 

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