Unstable nuclei

Despite their instability, some unstable atoms may last a long time the half-life of uranium 238, for example, is about 4.5 billion years. Other unstable atoms decay in a few seconds. Radioactive decay is one of the topics of nuclear chemistry, and it involves nuclear forces, as governed by advanced concepts in chemistry and physics, such as quantum mechanics. Researchers do not fully understand why some atoms are stable and others are not, but most radioactive nuclei have an unusually large (or small) number of neutrons, which makes the nucleus unstable. And all heavy nuclei found so far are radioactive—nuclides with an atomic number of 83 or greater decay. 

The ratio of protons to neutrons seems to determine the stability of the nucleus. Unstable nuclei can split up. 

ISeed nucleus Stable nucleus Unstable nucleus... 

Spontaneous fission (SF) is observed only in elements with Z> 90 where Coulomb forces make the nucleus unstable toward this mode of decay, although energetically SF is an exothermic process for nuclei with A > 100. Numerous reviews of SF properties, half-lives, and properties of fission fragments, have been summarized by several authors (von Gunten 1969 Hoffinan and Hoffinan 1974 Hoffinan and Somerville 1989 Hulet 1990b, Wagemans 1991 Hoffinan and Lane 1995 Hoffinan et al. 1996) and basic properties of nuclear fission are described in Chap. 4 of Vol. 1. However, some current topics concerning SF are presented in this Subsection. 

The most important types of radioactive particles are alpha particles, beta particles, gamma rays, and X-rays. An alpha particle, which is symbolized as a, is equivalent to a helium nucleus, fHe. Thus, emission of an alpha particle results in a new isotope whose atomic number and atomic mass number are, respectively, 2 and 4 less than that for the unstable parent isotope. 

Radioactivity occurs naturally in earth minerals containing uranium and thorium. It also results from two principal processes arising from bombardment of atomic nuclei by particles such as neutrons, ie, activation and fission. Activation involves the absorption of a neutron by a stable nucleus to form an unstable nucleus. An example is the neutron reaction of a neutron and cobalt-59 to yield cobalt-60 [10198 0-0] Co, a 5.26-yr half-life gamma-ray emitter. Another is the absorption of a neutron by uranium-238 [24678-82-8] to produce plutonium-239 [15117 8-5], Pu, as occurs in the fuel of a nuclear... 

The sulfated compounds MM 13902 (3, n = (5) and MM 17880 (4) are also broad-spectmm agents, but not as potent as thienamycia and all lack any significant activity against Pseudomonas (73). Many carbapenems are excellent inhibitors of isolated P-lactamases, particularly the olivanic acid sulfoxide MM 4550 (3, n = 1) (3). The possible mechanism of action of the carbapenems as inhibitors of P-lactamases has been discussed in some detail (74). Other carbapenems such as PS-5 (5) (75), the carpetimycins (76), asparenomycins (77), and pluracidomycins (8) are all highly active as antibiotics or P-lactamase inhibitors. The parent nucleus itself (1, X = CH2) is intrinsically active, but chemically unstable (9). 

Tritium [15086-10-9] the name given to the hydrogen isotope of mass 3, has symbol or more commonly T. Its isotopic mass is 3.0160497 (1). Moletecular tritium [10028-17-8], is analogous to the other hydrogen isotopes. The tritium nucleus is energetically unstable and decays radioactively by the emission of a low-energy P particle. The half-life is relatively short (- 12 yr), and therefore tritium occurs in nature only in equiUbrium with amounts produced by cosmic rays or man-made nuclear devices. 

This result has been plotted out in Fig. 7.1. It shows that there is a maximum value for Wjr corresponding to a critical radius r. For r < r (dV /dr) is positive, whereas for r > r it is negative. This means that if a random fluctuation produces a nucleus of size r < r it will be unstable the system can do free work if the nucleus loses atoms and r decreases. The opposite is true when a fluctuation gives a nucleus with r > r. Then, free work is done when the nucleus gains atoms, and it will tend to grow. To summarise, if random fluctuations in the liquid give crystals with r > r then stable nuclei will form, and solidification can begin. 

Decay The spontaneous disintegration of an unstable atomic nucleus to form another more stable element or isotope of a lower atomic mass. 

Unstable isotopes decompose (decay) by a process referred to as radioactivity. Ordinarily the result is the transmutation of elements the atomic number of the product nucleus differs from that of the reactant. For example, radioactive decay of produces a stable isotope of nitrogen, N. The radiation given off (Figure 2.6) may be in the form of—... 

Perhaps the most important first-order reaction is that of radioactive decay, in which an unstable nucleus decomposes (Chapter 2). Letting X be the amount of a radioactive isotope present at time t,... 

The products are an a-particle (a helium nucleus), and a thorium isotope that is unstable and that rapidly decays by emitting successively two electrons ... 

Thiophene 1,1-dioxide (61) is too unstable to isolate and dimerizes with loss of S02 to give 3a, 7a-dihydrobenzothiophene 1,1-dioxide (172) in 34%113. However, alkyl-substituted thiophene 1,1-dioxides can serve as dienes in the Diels-Alder reaction, since the aromatic properties of the thiophene nucleus are lost completely and the n-electrons of the sulfur atom are used for forming the bond with oxygen. The sulfones 173-178 are found to react with two moles of maleic anhydride at elevated temperature to give bicyclic anhydrides114. Thus, at high reaction temperature, S02 is split off to give cyclohexadiene... 

B.9 An unstable atomic nucleus gives off nuclear radiation consisting of particles that have a mass of about 1.7 X 10 kg. The particles are attracted to a negatively charged plate. The radiation consists of what type of subatomic particle ... 

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. 

The equation for the decay of a nucleus (parent nucleus - daughter nucleus + radiation) has exactly the same form as a unimolecular elementary reaction (Section 13.7), with an unstable nucleus taking the place of a reactant molecule. This type of decay is expected for a process that does not depend on any external factors but only on the instability of the nucleus. The rate of nuclear decay depends only on the identity of the isotope, not on its chemical form or temperature. 

When a nucleus is placed in a flux of neutrons, it may capture another neutron. It thus is often unstable toward further decay by j3 -emission. The induced radioactivity is critical to the study of chemical consequences of neutron capture, since so few of these new nuclei are produced that generally they cannot be observed by any other means. This radioactivity is not, however, a part of the phenomenon which we wish to observe and, moreover, is occasionally a distraction. 

For each different element, there are a few specific values of A that result in stable nuclei. Figure 2-20 shows all the stable nuclei on a plot of the number of neutrons (iV) versus the number of protons (Z). These data show a striking pattern All stable nuclei fall within a belt of stability. Any nucleus whose ratio of neutrons to protons falls outside the belt of stability is unstable and decomposes spontaneously. Lighter nuclei lie along the JV = Z line, but the N jZ ratio of stable nuclei rises slowly until it reaches 1.54. The trend is illustrated by the N j Z... 

When Z gets big enough, no number of neutrons is enough to stabilize the nucleus. Notice in Figure 2-20 that there are no stable nuclei above bismuth, Z — 83. Some elements with higher Z are found on Earth, notably radium (Z = 88), thorium (Z = 90), and uranium (Z = 92), but all such elements are unstable and eventually disintegrate into nuclei with Z < 83. Consequently, the set of stable nuclei, those that make up the world of normal chemistry and provide the material for all terrestrial chemical reactions, is a small subset of all possible nuclei. 

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