Elements stable nuclei 38

All elements of atomic number greater than 83 exhibit radioactive decay K, Rb, Ir and a few other light elements emit p particles. The heavy elements decay through various isotopes until a stable nucleus is reached. Known half-lives range from seconds to 10 years. 

More than 1500 radioactive isotopes have been prepared in the laboratory. The number of such isotopes per element ranges from 1 (hydrogen and boron) to 34 (indium). They are all prepared by bombardment reactions in which a stable nucleus is converted to one... 

Stars of mass greater than 1.4 solar masses have thermonuclear reactions that generate heavier elements (see Table 4.3) and ultimately stars of approximately 20 solar masses are capable of generating the most stable nucleus by fusion processes, Fe. The formation of Fe terminates all fusion processes within the star. Heavier elements must be formed in other processes, usually by neutron capture. The ejection of neutrons during a supernova allows neutron capture events to increase the number of neutrons in an atomic nucleus. Two variations on this process result in the production of all elements above Fe. A summary of nucleosynthesis processes is summarised in Table 4.4. Slow neutron capture - the s-process - occurs during the collapse of the Fe core of heavy stars and produces some higher mass elements, however fast or rapid neutron capture - the r-process - occurs during the supernova event and is responsible for the production of the majority of heavy nuclei. 

The stability of the atomic nucleus depends upon a critical balance between the repulsive and attractive forces involving the protons and neutrons. For the lighter elements, a neutron to proton ratio (N P) of about 1 1 is required for the nucleus to be stable but with increasing atomic mass, the N P ratio for a stable nucleus rises to a value of approximately 1.5 1. A nucleus whose N P ratio differs significantly from these values will undergo a nuclear reaction in order to restore the ratio and the element is said to be radioactive. There is, however, a maximum size above which any nucleus is unstable and most elements with atomic numbers greater than 82 are radioactive. 

Once a star s core temperature has reached about three billion degrees, fusion processes generate iron. And here they stop, because iron is the most stable nucleus of all. There is no energy to be gained by fusing iron nuclei. Yet heavier elements clearly do exist. They are created in the outer regions of the star, where neutrons emitted by fusion reactions are captured by nuclei to build all the elements up to bismuth (atomic number 73). 

While much of the preceding is speculative, it is no more speculative chemically than Mendeleev s predictions of gallium (eka-afuminum) and germanium (eka-silicon). The speculation centers on the possible or probable stability of nuclei with up to twice as many protons as the heaviest stable nucleus. The latter falls outside the realm of inorganic chemistry, but the synthesis and characterization of some of these elements would be most welcome. 

Very few nuclides with Z < 60 emit a particles. All nuclei with Z > 83 are unstable and decay mainly by a particle emission. Nuclides of elements with Z > 83 must discard protons to reduce their atomic number, and they generally need to lose neutrons too. These nuclei decay in a stepwise 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 P particle is ejected, and so on, until a stable nucleus is formed—usually the final nuclide is an isotope of lead (the element with the magic atomic number 82). 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 non-radioactive element, gO was first produced artificially by Ernest Rutherford in 1919. In order to achieve this, he bombarded stable nucleus with a-particles emitted by radium and polonium. 

Iron is the most stable nucleus of all. For elements with smaller atomic numbers than iron, fusion of nuclei to produce heavier elements releases energy, because the products are lighter and more stable on a per-nucleon basis than the reactants. In contrast, beyond iron, fusion absorbs energy because the products are heavier on a per-nucleon basis than the reactants. 

Radioactive Disintegration Series The series of spontaneous changes that take place starting from the parent element (which has unstable nucleus) till the formation of an element with stable nucleus is called... 

A number of additional observations indicate that there is extra stability associated with nuclei having an even number of protons, of neutrons, or, more especially, even numbers of both. Elements of odd atomic number have no more than two stable isotopes. Of these elements, about half exist as one stable nuclide only, and two have no stable forms. Aside from the four exceptions mentioned in the preceding paragraph, each stable nucleus having an odd number of protons must have an even number of neutrons. Among the natural radioactive series, about 20 nuclides of even Z, but only 2 of odd Z, have half-lives of over one day. 

All the nuclear reactions that have been described thus far are examples of radioactive decay, where one element is converted into another element by the spontaneous emission of radiation. This conversion of an atom of one element to an atom of another element is called transmutation. Except for gamma emission, which does not alter an atom s atomic number, all nuclear reactions are transmutation reactions. Some unstable nuclei, such as the uranium salts used by Henri Becquerel, undergo transmutation naturally. However, transmutation may also be forced, or induced, by bombarding a stable nucleus with high-energy alpha, beta, or gamma radiation. 

Our ability to detect minute amounts of radioisotopes makes them powerful tools for studying processes in biochemistry, medicine, materials science, environmental studies, and many other scientific and industrial fields. Such uses depend on the fact that isotopes of an element exhibit very similar chemical and physical behavior. In other words, except for having a less stable nucleus, a radioisotope has nearly the same chemical properties as a nonradioactive isotope of a given element. For example, the fact that " C02 is utilized hy a plant in the same way as "C02 forms the basis of radiocarbon dating. 

When the stellar core has been depleted of carbon and oxygen and is rich in silicon, the silicon-burning phase can begin, and silicon is converted to sulfur, argon, and other heavier elements. If contraction can raise the temperature of the interior to about 3 billion degrees, then the so-called equilibrium phase of the star s life cycle begins, and elements close to iron are formed. Iron is the most stable nucleus of all as noted, if a star were to burn to its end, it would become a ball of iron. 

Finally, the monocoordinated complexes can be expected to be present in the interstellar space and detected in radioastronomy provided the elemental abondance of the metal is sufficient. This should be the case of iron Fe, a very stable nucleus, the observation of which in molecules like FeCO [22] has failed up to now. Given the fact that cyano compounds formed by sodium and magnesium i.e., NaNC, MgNC and MgCN) have been already observed [23,24,25] calculations of iron complexes have some interest for Astrophysics. 

Eventually progressively heavier elements form at the core until it becomes predominantly Fe as shown in M FIGURE 21.23. Because this is such a stable nucleus, further fusion to heavier nuclei consumes energy rather than releasing it. When this happens, the fusion reactions that power the star diminish, and immense gravitational forces lead to a dramatic collapse called a supernova explosion. Neutron capture coupled with subsequent radioactive decays in the dying moments of such a star are responsible for the presence of all elements heavier than iron and nickel... 

---