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Origin of the Elements. Isotopes and Atomic Weights

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1 2 H He 3 4 5 6 7 8 9 10 Li Be B C N O F Ne 11 12 13 14 15 16 17 18 Na Mg Al Si PSCl Ar 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr YZrNb Mo Tc Ru Rh Pd Ag Cd ln Sn Sb Te IXe 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 87 88 89 104 105 106 107 108 109 110 111 112 Fr Ra Ac Rf Db Sg Bh Hs Mt Uun Uuu Uub 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 90 91 92 93 94 95 96 97 98 99 100 101 102 103 Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 1 Origin of the Elements. Isotopes and Atomic Weights 1.1 Introduction what is a currently acceptable theory which interprets the known facts. The tentative nature of This book presents a unified treatment of the our knowledge is perhaps nowhere more evident chemistry of the elements. At present 112 ele- than in the first few sections of this chapter ments are known, though not all occur in nature: dealing with the origin of the chemical elements of the 92 elements from hydrogen to uranium all and their present isotopic composition. This is not except technetium and promethium are found on surprising, for it is only in the last few decades earth and technetium has been detected in some that progress in this enormous enterprise has been stars. To these elements a further 20 have been made possible by discoveries in nuclear physics, added by artificial nuclear syntheses in the labo- astrophysics, relativity and quantum theory. ratory. Why are there only 90 elements in nature? Why do they have their observed abundances and why do their individual isotopes occur with the 1.2 Origin of the Universe particular relative abundances observed? Indeed, At present, the most widely accepted theory we must also ask to what extent these isotopic for the origin and evolution of the universe to abundances commonly vary in nature, thus caus- its present form is the “hot big bang”..1/ It ing variability in atomic weights and possibly is supposed that all the matter in the universe jeopardizing the classical means of determining chemical composition and structure by chemical 1 J. SILK, The Big Bang: The Creation and Evolution analysis. of the Universe, 2nd edn., W. H. Freeman, New York, Theories abound, and it is important at all 1989, 485 pp. J. D. BARROW and J. SILK, The Left Hand times to distinguish carefully between what has of Creation: The Origin and Evolution of the Expanding Universe, Heinemann, London, 1984, 256 pp. E. W. KOLB been experimentally established, what is a useful andM.S.TURNER, The Early Universe, Addison-Wesley, model for suggesting further experiments, and Redwood City, CA, 1990, 547 pp. 1 2 Origin of the Elements. Isotopes and Atomic Weights Ch. 1 was once contained in a primeval nucleus of of radiation would have immediately decomposed immense density (¾1096 gcm3) and temperature them back to protons and neutrons. Subsequently, (¾1032 K) which, for some reason, exploded the continuing expansion of the universe was and distributed radiation and matter uniformly such that the particle density was too low throughout space. As the universe expanded for these strong (but short-range) interactions it cooled; this allowed the four main types to occur. Thus, within the time slot of about of force to become progressively differentiated, eight minutes, it has been calculated that about and permitted the formation of various types one-quarter of the mass of the universe was of particle to occur. Nothing scientific can be converted to helium nuclei and about three- said about the conditions obtaining at times quarters remained as hydrogen. Simultaneously, 5 1/2 3 shorter than the Planck time, tP [.Gh/c / D a minute 10 % was converted to deuterons and 1.33 ð 1043 s] at which moment the forces of about 106% to lithium nuclei. These remarkable gravity and electromagnetism, and the weak and predictions of the big bang cosmological theory strong nuclear forces were all undifferentiated are borne out by experimental observations. and equally powerful. At 1043 s after the big Wherever one looks in the universe the oldest bang (T D 1031 K) gravity separated as a distinct stars in our own galaxy, or the “more recent” stars force, and at 1035 s(1028 K) the strong nuclear in remote galaxies the universal abundance of force separated from the still combined electro- helium is about 25%. Even more remarkably, weak force. These are, of course, inconceivably the expected concentration of deuterium has been short times and unimaginably high temperatures: detected in interstellar clouds. Yet, as we shall for example, it takes as long as 1024 sfor shortly see, stars can only destroy deuterium a photon (travelling at the speed of light) to as soon as it is formed; they cannot create traverse a distance equal to the diameter of an any appreciable equilibrium concentration of atomic nucleus. When a time interval of 1010 s deuterium nuclei because of the high temperature had elapsed from the big bang the temperature of the stellar environment. The sole source of is calculated to have fallen to 1015 K and this deuterium in the universe seems to be the big enabled the electromagnetic and weak nuclear bang. At present no other cosmological theory forces to separate. By 6 ð 106 s .1.4 ð 1012 K/ can explain this observed ratio of H:He:D. protons and neutrons had been formed from Two other features of the universe find quarks, and this was followed by stabilization ready interpretation in terms of the big bang of electrons. One second after the big bang, theory. First, as observed originally by E. Hubble after a period of extensive particle antiparticle in 1929, the light received on earth from annihilation to form electromagnetic photons, distant galaxies is shifted increasingly towards the universe was populated by particles which the red end of the spectrum as the distance sound familiar to chemists protons, neutrons of the source increases. This implies that and electrons. the universe is continually expanding and, on Shortly thereafter, the strong nuclear force certain assumptions, extrapolation backwards ensured that large numbers of protons and in time indicates that the big bang occurred neutrons rapidly combined to form deuterium some 15 billion years ago. Estimates from nuclei (p C n), then helium (2p C 2n). The several other independent lines of evidence process of element building had begun. During give reassuringly similar values for the age of this small niche of cosmic history, from about the universe. Secondly, the theory convincingly 10 500 s after the big bang, the entire universe explains (indeed predicted) the existence of is thought to have behaved as a colossal an all-pervading isotropic cosmic black-body homogeneous fusion reactor converting hydrogen radiation. This radiation (which corresponds to a into helium. Previously no helium nuclei could temperature of 2.735 š 0.06 K according to the exist the temperature was so high that the sea most recent measurements) was discovered in §1.3 Abundances of the elements in the universe 3 1965 by A. A. Penzias and R. W. Wilson.2/ and (i) Abundances decrease approximately is seen as the dying remnants of the big bang. No exponentially with increase in atomic other comological theory yet proposed is able to mass number A until A ¾ 100 (i.e. Z ¾ interpret all these diverse observations. 42); thereafter the decrease is more grad- ual and is sometimes masked by local fluctuations. 1.3 Abundances of the (ii) There is a pronounced peak between Z D 23 28 including V, Cr, Mn, Fe, Co and Elements in the Universe Ni, and rising to a maximum at Fe which is ¾103 more abundant than expected Information on the abundances of at least some from the general trend. of the elements in the sun, stars, gaseous (iii) Deuterium (D), Li, Be and B are rare nebulae and the interstellar regions has been compared with the neighbouring H, He, obtained from detailed spectroscopic analysis C and N. using various regions of the electromagnetic (iv) Among the lighter nuclei (up to Sc, Z D spectrum. This data can be supplemented by 21), those having an atomic mass number direct analysis of samples from the earth, from A divisible by 4 are more abundant than meteorites, and increasingly from comets, the their neighbours, e.g. 16O, 20Ne, 24Mg, moon, and the surfaces of other planets and 28Si, 32S, 36Ar and 40Ca (rule of G. Oddo, satellites in the solar system. The results indicate 1914). extensive differentiation in the solar system and (v) Atoms with A even are more abundant in some stars, but the overall picture is one of than those with A odd. (This is seen in astonishing uniformity of composition. Hydrogen Fig. 1.1 as an upward displacement of is by far the most abundant element in the the curve for Z even, the exception at universe, accounting for some 88.6% of all beryllium being due to the non-existence atoms (or nuclei).
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