SECTION 2 Nuclear Change

None of the reactions or processes stndied in previous chapters affected the nucleus of an atom. No atom changed from one element to another. This chapter considers the effects of nuclear change. In most cases, such changes cause a transformation from one element to another. They include the natural radioactivity of certain isotopes (Section 3.3), as well as the artificial nuclear reactions discovered during the twentieth century. Nuclear reactions differ from ordinary chemical reactions in the following ways ... 

Half-Life of Radioactive Decay Decay rates are also commonly expressed in terms of the fraction of nuclei that decay over a given time interval. The half-life (fi/i) of a nuclide is the time it takes for half the nuclei present in a sample to decay. The number of nuclei remaining is halved after each half-life. Thus, half-life has the same meaning for a nuclear change as for a chemical change (Section 16.4). Figure 23.4 shows the decay of carbon-14, which has a half-life of 5730 years, in terms of number of C nuclei remaining ... 

Interestingly, the apoptotic activity of the nuclear assembly extracts had an absolute requirement for a subcellular fraction highly enriched in mitochondria. The system was used to test the effect of various treatments on apoptosis and it was found that the observed nuclear changes were inhibited by the addition of, amongst other things, inhibitors of calpain (a cysteine protease see Section 7). 

Provided that we may have x, different from those of the uncoupled system, we should still take into consideration that the interaction energy between the unpaired electron on a metal ion and a nucleus sensing it changes if the metal ion is magnetically coupled to another metal ion, as discussed in the previous section. Nuclear relaxation depends on the square of the above energy [1], For a monomeric system the contribution to nuclear relaxation due to the presence of unpaired electron(s) can be described by the general equation ... 

The analysis of steady-state and transient reactor behavior requires the calculation of reaction rates of neutrons with various materials. If the number density of neutrons at a point is n and their characteristic speed is v, a flux effective area of a nucleus as a cross section O, and a target atom number density N, a macroscopic cross section E = Na can be defined, and the reaction rate per unit volume is R = 0S. This relation may be appHed to the processes of neutron scattering, absorption, and fission in balance equations lea ding to predictions of or to the determination of flux distribution. The consumption of nuclear fuels is governed by time-dependent differential equations analogous to those of Bateman for radioactive decay chains. The rate of change in number of atoms N owing to absorption is as follows ... 

What Do We Need to Know Already Nuclear processes can be understood in terms of atomic structure (Section B and Chapter 1) and energy changes (Chapter 6). The section on rates of radioactive decay builds on chemical kinetics (particularly Sections 13.4 and 13.5). 

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. 

The premise of this review is that synthetic procedures for very mixed"-metal clusters are comparatively well understood, but that reactivity and physical properties are less well studied. Metal core transformations (modifications of a preexisting cluster) fall into both the synthesis and reactivity categories. A summary is presented here, but as they have been reviewed elsewhere (see Refs. 4, 107-109), the account below is necessarily brief. Section lI.E. 1. considers core transformations where the cluster core nuclearity is pre.served, whereas Section 11.E.2. summarizes reactions involving a change in core size. 

In the preceding section, the interaction energy between two reacting molecules has been discussed with the assumption of no nuclear configuration change. In the donor-acceptor interaction the delocalization stabilization is dominant. Eq. (3.25) indicates the importance of HO and LU in the donor-acceptor interaction. But the expression of Eq. (3.21) shows that in general cases the contribution of HO and LU to the quantity D is not so discriminative as those of the other MO s. 

Renal Effects. The characteristics of early or acute lead-induced nephropathy in humans include nuclear inclusion bodies, mitochondrial changes, and cytomegaly of the proximal tubular epithelial cells dysfunction of the proximal tubules (Fanconi s syndrome) manifested as aminoaciduria, glucosuria, and phosphaturia with hypophosphatemia and increased sodium and decreased uric acid excretion. These effects appear to be reversible. Characteristics of chronic lead nephropathy include progressive interstitial fibrosis, dilation of tubules and atrophy or hyperplasia of the tubular epithelial cells, and few or no nuclear inclusion bodies, reduction in glomerular filtration rate, and azotemia. These effects are irreversible. The acute form is reported in lead-intoxicated children, whose primary exposure is via the oral route, and sometimes in lead workers. The chronic form is reported mainly in lead workers, whose primary exposure is via inhalation. Animal studies provide evidence of nephropathy similar to that which occurs in humans, particularly the acute form (see Section 2.2.3.2). 

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