Nuclides Numbers

The splitting of levels leads to an increased level density. Energetic gaps ( closed shells ) found in a spherical nucleus may disappear in a deformed nucleus. New gaps may be formed at different nuclide numbers at specific deformations. The sum of the energetic effects of all the occupied quantum states in a nucleus shows an oscillating behavior, i.e., the stability of a nucleus (as far as shells are concerned) as a function of deformation shows maxima and minima. 

Nuclide. Each nuclide is identified by element name and the mass number A, equal to the sum of the numbers of protons Z and neutrons N in the nucleus. The m following the mass number (for example, Zn) indicates a metastable isotope. An asterisk preceding the mass number indicates that the radionuclide occurs in nature. Half-life. The following abbreviations for time units are employed y = years, d = days, h = hours, min = minutes, s = seconds, ms = milliseconds, and ns = nanoseconds. 

Several portions of Section 4, Properties of Atoms, Radicals, and Bonds, have been significantly enlarged. For example, the entries under Ionization Energy of Molecular and Radical Species now number 740 and have an additional column with the enthalpy of formation of the ions. Likewise, the table on Electron Affinities of the Elements, Molecules, and Radicals now contains about 225 entries. The Table of Nuclides has material on additional radionuclides, their radiations, and the neutron capture cross sections. 

Our present views on the electronic structure of atoms are based on a variety of experimental results and theoretical models which are fully discussed in many elementary texts. In summary, an atom comprises a central, massive, positively charged nucleus surrounded by a more tenuous envelope of negative electrons. The nucleus is composed of neutrons ( n) and protons ([p, i.e. H ) of approximately equal mass tightly bound by the force field of mesons. The number of protons (2) is called the atomic number and this, together with the number of neutrons (A ), gives the atomic mass number of the nuclide (A = N + Z). An element consists of atoms all of which have the same number of protons (2) and this number determines the position of the element in the periodic table (H. G. J. Moseley, 191.3). Isotopes of an element all have the same value of 2 but differ in the number of neutrons in their nuclei. The charge on the electron (e ) is equal in size but opposite in sign to that of the proton and the ratio of their masses is 1/1836.1527. 

Since the radioactive half-lives of the known transuranium elements and their resistance to spontaneous fission decrease with increase in atomic number, the outlook for the synthesis of further elements might appear increasingly bleak. However, theoretical calculations of nuclear stabilities, based on the concept of closed nucleon shells (p. 13) suggest the existence of an island of stability around Z= 114 and N= 184. Attention has therefore been directed towards the synthesis of element 114 (a congenor of Pb in Group 14 and adjacent superheavy elements, by bombardment of heavy nuclides with a wide range of heavy ions, but so far without success. 

Element has no stable nuclides the value given in parentheses is the atomic mass number of the isotope of longest known half-life. However, three such elements (Th, Pa and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated. 

The discoveries of Becquerel, Curie, and Rutherford and Rutherford s later development of the nuclear model of the atom (Section B) showed that radioactivity is produced by nuclear decay, the partial breakup of a nucleus. The change in the composition of a nucleus is called a nuclear reaction. Recall from Section B that nuclei are composed of protons and neutrons that are collectively called nucleons a specific nucleus with a given atomic number and mass number is called a nuclide. Thus, H, 2H, and lhO are three different nuclides the first two being isotopes of the same element. Nuclei that change their structure spontaneously and emit radiation are called radioactive. Often the result is a different nuclide. 

STRATEGY Write the nuclear equation for each reaction, representing the daughter nuclide as E, with atomic number Z and mass number A. Then find Z and A from the requirement that both mass number and atomic number are conserved in a nuclear reaction, (a) In a decay, two protons and two neutrons are lost. As a result, the mass number decreases by 4 and the atomic number decreases by 2 (see Fig. 17.7). (b) The loss of one negative charge when an electron is ejected from the nucleus (Fig. 17.8) can be interpreted as the conversion of a neutron into a proton within the nucleus ... 

Figure 17.13 is a plot of mass number against atomic number for known nuclides. Stable nuclei are found in a band of stability surrounded by a sea of instability, the region of unstable nuclides that decay with the emission of radiation. For atomic numbers up to about 20, the stable nuclides have approximately equal numbers of neutrons and protons, and so A is close to 2Z. For higher atomic numbers, all known nuclides—both stable and unstable—have more neutrons than protons, and so A > 2Z. 

FIGURE 17.12 The numbers of stable nuclides having even or odd numbers of neutrons and protons. With the exception of hydrogen, by far the greatest number of stable nuclides (157) have even numbers of both protons and neutrons. Only four stable nuclides have odd numbers of both protons and neutrons. 

Very few nuclides with Z < 60 emit a particles. All nuclei with Z > 82 are unstable and decay mainly by a-particle emission. They must discard protons to reduce their atomic number and generally need to lose neutrons, too. These nuclei decay in a step-by-step manner and give rise to a radioactive series, a characteristic sequence of nuclides (Fig. 17.16). First, one a particle is ejected, then another a particle or a (3-particle is ejected, and so on, until a stable nucleus, such as an iso tope of lead (with the magic atomic number 82) is formed. For example, the uranium-238 series ends at lead-206, the uranium-235 series ends at lead-207, and the thorium-232 series ends at lead-208. 

A large number of nuclides have been synthesized on Earth. For instance, technetium was prepared (as technetium-97) for the first time on Earth in 1937 by the reaction between molybdenum and deuterium nuclei ... 

Mt, Z = 109) were formally named in 1997. The transmeitnerium elements, the elements beyond meitnerium (including hypothetical nuclides that have not yet been made) are named systematically, at least until they have been identified and there is international agreement on a permanent name. Their systematic names use the prefixes in Table 17.2, which identify their atomic numbers, with the ending -him. Thus, element 110 was known as ununnilium until it was named darmstadtium (Ds) in 2003. 

Information on isochronal annealing of Mo(CO)g has been given recently by Groening and Harbottle The most interesting result in this work was the clearly stepwise nature of the annealing, as is shown in Fig. 6. Curiously, not only the retention values but also the number and positions of the steps show isotropic differences. No clear explanation was offered other than the suggestion that the effect must arise from differences in the decay modes of the two excited nuclides. 

Formation of labeled molecules has been studied in a few cases, but has not been exploited usefully. Various radioactive organomercury compounds have been prepared diphenylmercury (33, 90), fluorescein (53), and chloromeredrin (43). A number of other potentially useful syntheses could doubtless be developed with a wide variety of nuclides with easily detectable y-rays—pharmaceuticals, pesticides, physiological tracers, oil-soluble markers for labeling oil shipments, and so on—if it could be established what molecules are of interest to the various consumers ... 

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... 

As described in Chapter 2, the mass number of a nuclide. A, is its total number of protons and neutrons ... 

Cu). Alternatively, the name of the element is followed by its mass number, as in copper-63. Example provides some practice in writing the symbols of nuclides. 

When determining symbols for nuclides, the key is to remember that the atomic number and number of protons are the same and that the mass number is the sum of the number of protons plus the number of neutrons. 

C22-0001. W rite the nuclear symbols for the following nuclides (a) the nuclide that contains 15 protons and 16 neutrons (b) neon with the same number of neutrons as protons. 

The tabulated molar mass of an element divided by Avogadro s number is the average mass per atom of that element, but it is not the exact mass of an individual nucleus. There are two reasons for this. First, molar masses refer to neutral atoms. The tabulated molar mass of an element includes the mass of its electrons in addition to the mass of its nucleus. Consequently, the mass of Z electrons must be subtracted from the isotopic molar mass in computing the energy of formation of a nuclide. Second, molar masses of the elements are weighted averages of... 

Plot of the binding energy per nucleon vs. mass number A. The most stable nuclides lie in the region around... 

Although nuclides with mass numbers around 60 are the most stable, the balance of electrical repulsion and strong nuclear attraction makes many combinations of protons and neutrons stable for indefinite times. Nevertheless, many other combinations decompose spontaneously. For example, all hydrogen nuclides with j4 > 2 are so... 

As described in Chapter 2 (see Figure ), stable nuclides fall within a belt of stability with roughly equal numbers of neutrons and protons. Lighter nuclides lie along the = Z line, but as the mass of the nuclide increases, the... 

Within the belt of stability. Table 22-2 shows that nuclides with even numbers of protons and neutrons are more prevalent than those with odd numbers of protons or neutrons. Almost 60% of all stable nuclides have even numbers of both protons and neutrons, whereas fewer than 2% have odd numbers of both. Moreover, of the five... 

To summarize, the equation for a nuclear reaction is balanced when the total charge and total mass number of the products equals the total charge and total mass number of the reactants. This conservation requirement is one reason why the symbol for any nuclide includes its charge number (Z) as a subscript and its mass number as a superscript. These features provide a convenient way to keep track of charge and mass balances. Notice that in the equation for neutron decay, the sum of the subscripts for reactants equals the sum of the subscripts for products. Likewise, the sum of the superscripts for reactants equals the sum of the superscripts for products. We demonstrate how to balance equations for other reactions as they are introduced. 

Charge number and mass number must be conserved in each reaction. Thus, each a particle decreases the nuclear charge by two units and the mass number by four units. Similarly, each P emission increases the nuclear charge by one unit but leaves the mass number unchanged. Consult a periodic table to identify the elemental S3Tnbol of each product nuclide. 

Fission follows neutron capture by a small number of the heaviest nuclides, notably U, Pu, and... 

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