FORMATION OF THE HEAVY ELEMENTS

The formation of most of the heavy elements occurs in one of two processes of neutron capture the s-process or the r-process. These two broad divisions are distinguished on the basis of the relative lifetimes for neutron captures (Xn) and electron decays (t ). The condition that t > where Tp is a characteristic lifetime for /3-unstable nuclei near the valley of /3-stability, ensures that as captures proceed the neutron-capture path will itself remain close to the valley of /3-stability. This defines the s-process. In contrast, when it follows that successive neutron captures will proceed into the neutron-rich regions off the /3-stable valley. Following the exhaustion of the neutron flux, the capture products approach the position of the valley of /3-stability by /3-decay, forming the r-process nuclei. The s-process and r-process patterns in solar system matter are those shown in Figure 2. 

Thus, chemical experiments must be carefully chosen with respect to a specific property which is strongly influenced by relativistic effects such that major changes could be detected [41]. The heat of formation of a heavy-element... 

When all the nuclear fuel is consmned, the core collapses in a very short time, followed by an explosion of the star in the form of a supernova. The explosion and the conditions generated by it lead to the formation of many heavy elements and the expulsion of this material into space. All this takes place in the outer parts of the star, while the inner core undergoes an implosion or collapse. During the collapse phase, the nuclei of iron are broken down to form neutrons and the entire star forms a neutron star in cases where the mass is up to two to three solar masses. In heavier stars, not even the Pauli exclusion principle can halt the further collapse of the star to form a black hole. ... 

As already mentioned, an alternative theory postulates a gravitational collapse of accretion disk material without first forming planetary cores (Cameron, 1978). In this case one would expect all of the outer planets to contain elements in nearly the same proportions, very similar to that of the Sun. Small increases of heavy elements over solar abundance ratios are possible from a post-accumulation infall of planetesimals, but the large enrichment of some of the heavy elements, as indicated in Table 9.3.1, caimot be explained. In contrast, the theory suggesting core formation to have occurred before the bulk of the low-Z gases arrived readily explains such an enrichment. 

The cobalt complex is usually formed in a hot acetate-acetic acid medium. After the formation of the cobalt colour, hydrochloric acid or nitric acid is added to decompose the complexes of most of the other heavy metals present. Iron, copper, cerium(IV), chromium(III and VI), nickel, vanadyl vanadium, and copper interfere when present in appreciable quantities. Excess of the reagent minimises the interference of iron(II) iron(III) can be removed by diethyl ether extraction from a hydrochloric acid solution. Most of the interferences can be eliminated by treatment with potassium bromate, followed by the addition of an alkali fluoride. Cobalt may also be isolated by dithizone extraction from a basic medium after copper has been removed (if necessary) from acidic solution. An alumina column may also be used to adsorb the cobalt nitroso-R-chelate anion in the presence of perchloric acid, the other elements are eluted with warm 1M nitric acid, and finally the cobalt complex with 1M sulphuric acid, and the absorbance measured at 500 nm. 

Reaction of cyclic tetrasulfido complexes of heavier group 14 elements bearing bulky substituents such as Tbt(Ar)MS4 (M=Si, Ar=Tip M=Ge, Ar=Tip M=Sn, Ar=Ditp) with 3 equivalents of phosphines afforded the successful isolation of the first stable double-bonded compounds between heavier group 14 elements and sulfur atom (heavy ketones), Tbt(Ar)M=S, accompanied by the quantitative formation of the corresponding phosphine sulfides (Scheme 40) [13, 15, 112, 113]. On the other hand, their lead an-... 

The examples in Table III, show that the hydrogen atoms occupy tetrahedral holes at the beginning of the transition series. As we move along the transition series, we observe the interstitial hydride shift toward octahedral holes and the hydrides of the heavier elements become progressively unstable. Palladium is exceptional since it is the only heavy element of group VIII that gives a simple hydride. Hydride formation is accompanied in most cases by a change in metallic lattice type and in all cases by a considerable increase in metal-metal distances. 

The half-life of 244Pu (8.2 X 107 years) is short compared with the age of the earth (4.5 X 109 years), and hence this nuclide is now extinct. However, the time interval (a) between the element synthesis in stars and formation of the solar system may have been comparable with the half-life of 244Pu. It has been found recently in this laboratory that various meteorites contain excess amounts of heavy xenon isotopes, which appear to be the spontaneous fission decay products of 244Pu. The value of H calculated from the experimental data range between 1 to 3 X 108 years. The process of formation of the solar system from the debris of supernova is somewhat analogous to the formation of fallout particles from a nuclear explosion. 

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