P.A. Cox - Inorganic chemistry (793955), страница 58
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Complex solution equilibriaare possible: with Pu, for example, all states from +3 to +6 can be present simultaneously. The different redox stabilityof U and Pu is important in nuclear fuel reprocessing, one function of which is to separate unused uranium from252I2—ACTINIUM AND THE ACTINIDESFig. 1. Frost diagram showing the oxidation states of some actinides in aqueous solution at pH=0.239Pu,which is itself used as a nuclear fuel. Dissolving the spent fuel elements in aqueous HNO3 gives PuIV and UVI.Subsequent separation steps then depend on differences in complexing power and solubility of these ions.The organometallic chemistry is much less extensive than that of the d block (see Topic H10), and differs fromthat of the lanthanides by virtue of the large sizes of the early actinides, and their wider range of accessible oxidationstates.
Uranium has been much more investigated than other elements. Typical compounds include thecyclopentadienyl (Cp=η5−C5H5) compounds [AnCp3], [AnCp4] and mixed Cp-halides such as [AnCp3Cl]. Particularlyinteresting is the sandwich compound [U(η8−C8H8)2] with two planar cyclooctatetraene rings known as uranocene;analogs are formed with neighboring actinides. Although formally they can be regarded as compounds of An4+ with thearomatic 10 π electron ring [C8H8]2− (see Topic C6) there is some covalent bonding involving actinide 5f and 6dorbitals.Later actinides show a much more restricted range of oxidation states, and are more similar to the lanthanides.The +4 state is found in AnO2 and AnF4 as far as Cf.
It becomes progressively more oxidizing for later elements, butwith a break at Bk4+ (which is more easily formed than Cm4+ or Cf4+) following the half-filled 5f shell and so analogousto the occurrence of Tb4+ in the lanthanides.
From Am to Md the +3 state is most stable in solid compounds andaqueous solution. Near the end of the series, however, the +2 state appears more stable than in the lanthanides and isthe normal one for No. This difference must reflect a different balance of ionization energies and lattice or solvationenergies, but the data required to understand it in detail are not available.
With only a few atoms available, and withvery short half-lives, chemical investigations of later actinides depend on tracer techniques using a stable element ofpresumed similar chemical behavior to act as a carrier. For example, the presence of No2+ can be inferred from itsprecipitation (and subsequent detection by its radioactivity) along with Ba2+ as BaSO4 under conditions where otheroxidation states form soluble compounds.Section J—Environmental, biological and industrial aspectsJ1ORIGIN AND ABUNDANCE OF THE ELEMENTSKey NotesPatterns of abundanceThe originelementsoftheFormation of the EarthRelated topicsO and Si are the commonest elements in the Earth’s crust, Fe and Oin the Earth as a whole, and H and He in the Solar System and theUniverse.
Nuclear reactions (controlling the amounts of elementsmade) and subsequent chemical reactions are both important indetermining these abundances.The lightest elements H and He were formed at the origin of theUniverse. Nearly all others have been made by nuclear fusionreactions inside stars. Fusion of He makes C and O, and then heavierelements up to Fe. Elements heavier than Fe are formed by neutronbombardment of lighter nuclei.
Nuclei with even atomic and/or massnumbers tend to be commoner than those with odd ones.The Earth was formed from solid dust particles containing metallicelements such as iron, together with silicates and other solids. Theabundant elements on Earth are ones that are both made in largequantities in nuclear reactions, and also condensed efficiently to formsolids.The nuclear atom (A1)Geochemistry (J2)Patterns of abundanceInformation on the abundance of elements comes from diverse sources. Most elements are obtained from minerals inthe Earth’s crust. The availability of elements therefore depends on the crustal abundance, which can be estimatedby analyzing representative samples of minerals.
The abundances of elements vary enormously, from common ones suchas oxygen and silicon (respectively 46% and 27% by mass) down to ones such as Os, Ir and Xe (one part in 1010 orless). The commonest elements are listed in Table 1.The crust is thin, and rests on the Earth’s mantle, which in turn surrounds the metallic core. As these inner regionsare not directly accessible, information on their composition comes from less direct sources, including meteorites, whichfall from space, and which are derived from one or more planets that broke up in the early stages of formation of theJ1—ORIGIN AND ABUNDANCE OF THE ELEMENTS255Table 1.
The most abundant elements in the crust, the whole Earth and the Solar System (mass fraction, with elements listed in order of decreasingabundance within each range)Solar System. Estimates of the overall abundance of elements in the whole Earth show some differences from the crust(see Table 1). Iron is the dominant element in the core and has a similar abundance to oxygen in the Earth as a whole.The Solar System is dominated in mass by the Sun. Estimates of elemental composition can be obtained from thespectrum of sunlight, which shows atomic absorption lines. Hydrogen and helium are by far the most abundantelements, followed at a level of less than 1% by oxygen and carbon. This pattern of abundances is typical of theUniverse as a whole, which is dominated by H and He in an atomic ratio of about 10:1, all other elements togethermaking up only 1%.Two very different factors are important in determining the abundance patterns shown in Table 1.
The overallabundance in the Universe and in the Solar System depends on how elements were made by nuclear reactions. Thevery different distribution in the Earth and its constituent parts is a consequence of subsequent chemicaldifferentiation of elements during the formation of the planets.The origin of the elementsThe synthesis of elements requires nuclear reactions, of which the most important type is the fusion of two lightnuclei to make one of higher charge and mass. The attractive strong interaction, which holds protons and neutronstogether, operates only over very short distances (around 10−15 m) and is opposed at longer range by the electrostaticrepulsion between positively charged protons.
To get two nuclei close enough together for fusion requires enormouslyhigh energies, which are normally found only at extreme temperatures (above 107 K) in the interior of stars. Undersuch conditions the chemical properties of elements are irrelevant, as no compounds can exist, atoms being in highlyionized states stripped of their electrons.It is thought that the Universe began about 15 billion years ago in a state of extraordinarily high temperature andpressure known as the big bang.
It rapidly cooled, and exotic elementary particles originally present formed protons,neutrons and electrons. Some protons and neutrons combined to form nuclei of deuterium (2H, the heavy isotope ofhydrogen; see Topic F2), which then fused to form 4He nuclei. Because of the rapidly falling temperature nuclearreactions ceased after about 3 min, and only very tiny amounts of elements heavier than helium were formed.Calculations based on the assumed conditions agree very well with the observed abundance of hydrogen and helium inthe Universe. The dominance of these elements forms one of the strongest pieces of evidence for the big bang model.As hydrogen and helium cooled, local gas concentrations formed and contracted under gravitational forces. Release ofgravitational potential energy heated the center of each concentration to the temperature (around 107 K) where nuclearfusion reactions restarted.
The energy output of all stars, including our Sun, comes from such reactions. Fusion ofhydrogen nuclei produces helium, and forms the energy source for stars throughout most of their lifetime. Whenhydrogen is used up in the center of a star, further gravitational contraction raises the temperature to about 108 K and4He nuclei themselves start to fuse. The main products of this stage are 12C and 16O, the most abundant nuclei in theUniverse after H and He. Exhaustion of He gives higher temperatures and further fusion reactions, producing elementsup to around iron. 56Fe has the highest binding energy of all nuclei, fusion reactions producing heavier nuclei beingendothermic.
Elements such as Co and Ni just beyond Fe are produced in equilibrium at the enormously hightemperatures (above 109 K) at the center of a star in the final stages of its life, but beyond this point successive elements256SECTION J—ENVIRONMENTAL, BIOLIGICAL AND INDUSTRIAL ASPECTSare formed by a process of neutron capture. Neutrons are produced as side products of some of the fusion reactions.They may be captured by nuclei, followed by a radioactive β decay process, which leads to an element of higher atomicnumber (see Topic A1).
Successive capture and decay processes are thought to have produced all the heavy elements,probably including some transuranium elements (see Topic J2) that have subsequently decayed.When no further exothermic nuclear reactions are possible in the center of a star, it collapses under gravitationalattraction, which releases enough energy to cause a gigantic explosion known as a supernova, which throws most ofthe material ‘cooked’ by nuclear reactions into space. Studies of supernovae in nearby galaxies show atomic spectrallines confirming the presence of these elements.Calculations based on these ideas can account for the abundance of elements, and of their different isotopes, observedin the Universe.
The nuclei made in greatest numbers are the most stable ones, generally having even numbers ofprotons and neutrons. Beyond 12C and 16O the most abundant are 20Ne, 24Mg, 28Si, 32S and 56Fe. For this reason (whichhas nothing to do with chemistry) elements in odd-numbered groups in the periodic table tend to be less common thanin even-numbered ones, a pattern that is apparent in the composition of the whole Earth shown in Table 1.Formation of the EarthGases thrown out by a supernova cool, and may subsequently be incorporated into new stars.
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