J. Rvdbera Department of Nuclear chemistry Chalmers University of Technology Goteborg S-Sweden G. R. Choppin Florida State University Tallahassee, 32306
I Elemental Evolution and Isotopic Composition
The creation of the universe is a topic of wonder and fascination to students of all abilities and interests. Too few teachers in freshman chemistry take advantage of this interest. Moreover, the processes of elemental creation are worth teaching not only because of the interest they hold for the student, but also because they deal with science in its broadest and most fundamental aspects (1-3). Selbin (4) recently described these processes in some detail and his articles can provide the basis for several lectures. Beyond the formation of the elements, an additional lecture could describe the chemical and physical bases for the particular elemental composition of the planets in our solar system. Such a lecture allows considerable scope to the teacher in illustrating the role of chemical principles in shaping the planets and in making earth so uniauelv . . suited for life. \Ve suspect that many teachers omit these topics because they feel that they are so intrinsicallv comnlex rhar anv discussion must be too sophisticated forman;of their students. We disagree with this view and oronose in this article arather simple approach to a presentatibn of elemental creation. This approach makes use of the assumption that the isotooic composition of the chemical elements provides clues to the processes which took place a t the dawn of time. Astronomy tells us that we are living in an expanding universe about 20 billion years old. The expansion may continue forever, or may cease and change into a contraction of all matter. Under any circumstances, it will take some multiples of 20 X lo9 yr before a final collapse occurs (if ever). Thus, from this standpoint, our universe is still young, and our assumption that we still can find some remnants of the birth of the universe in its elemental abundances and the isotopic composition seems reasonable. Elemental formation has continued in the stars since the original first formation and this is included in our consideration of the abundancies and isotonic com~osition. The nhundancies of the elements in the universe can he found in tnnn\. references (e.e. r 71 whilc the isoto~iccomposition of the elements, as f&nd on earth and whilh we assume to be representative of the solar system, can be found in 'isotope charts' (8) and tables (9).About 90% of the atoms in the universe are hvdro~en,and most of the rest are helium. All the other elemen& co&itute only 0.1% of the atoms of the universe. Because they are the most abundant and the simplest atoms, hydrogen atoms (or rather nuclei) are assumed to be the fundamental building blocks of matter. This was proposed first in 1816 by the English physician William Proust when he observed that most of the atomic weights measured a t that time were multiples of the atomic weight of the hydrogen atom. In 1948 Georg Gamow and coworkers advanced the hypothesis that the truly original material of the universe was not hydrogen (or, protons) hut neutrons. According to their theory about 20 billion years ago these originalneutrons underwent rapid radioactive decay to protons with the release of an enormous amount of energy. This energy release gave big bang theory." In the'big hang,' their theory its nam*"the ~
~~
'Neutrons and protons are known collectively as nucleons so the mass number, A , of a nucleus, is the number of nucleons or A = N + Z where N is the neutron number and Z, the proton number. Neutrons and protons have n mass close to one and the atomic weight should be close to an integral value. The fact that some atomicweights have values which are far from interval led to the discovery of isotopes. 742 1 Journal of Chemical Education
1234
6 7
1122
33
0112
34
9 11 13 15 17 19 21 23 25 27 29 31 33 35 10 12 14 16 18 2 0 22 24 26 2 8 30 32 34 4 5 5 6 6 7 7 8 8 8 9 10 11 12 13 14 15 16 17 10 10 12 12 14 14 15 16
A
5 5 6 6 7 7 8 8 9 10 11 12 1314 15 16 1718 10 10 12 12 14 14 16 16 18
Figure 1. The abundancies of stable nuclei with mass number A up to 35. The lower row gives me atomic number 2,to which lhe parlicular mass number belongs. No nuclei of A = 5 and 8 are stable ( 10). elements were created through combination of the primordial neutrons and protons into more complex nuclei. Although the relation A = N + Z is simple,' it tells us nothing about the processes in which elements of higher A were built up from the primordial neutrons and protons. In Figure 1we present a bar graph of the abundancies of stahle nuclides expressed as a Dercentaee of their isotonic abundance (i.e., % of the nuclei of differentmass numbers; A, with same Z) as a function of mass number up to A = 35. We can note several clues to the processes of eiemental formation from Figure 1. First we see that u p to A = 35, only one particular combination of Z and N provides a stahle nucleus for each A-in other words, only a single isobar is stahle for each A value. For example, for A = 14, the isobar of N = 7 and Z = 7 has 100% abundance; isobars of Z = 6 and N = 8, Z = 8 and N = 6, etc. are unstable. Moreover, some values of A are associated with stable species of low isotopic abundances; e.g., 2His 0.015% of hydrogen and 3He is 1.3 X of helium. For two values of A, 5 and 8, no stahle isobars exist, a fact to which we return shortly. The second observation of Figure 1is that high abundance nuclei seem to be seoarated from each other hv,a mass numher of 4. This regular pattern indicntrs that 4He may be an important entity in buildine un the liehter nuclei hv H renrtion in which 4 ~ e - i added s t o a nucleus. For example, this could lead t o reactions such as ~~~~~~
-
-
k2C+:He
hfiO
~
(1)
A60+ :He b9F + iH (2) The fact that 4He is very abundant in the universe must indicate that direct interactions hetween neutrons and protons a t the time of the "big hang" formed 4He. Let us consider the problem of the nonexistence of stable nuclei with A = 5 and A = 8. If the elements had heen ~-~hnilt. - ~ ~ up only through proton (or neutron) addition, it would have been imoossihle to oass even the first harrier of A = 5. However, with sttlhle nuclei of masses l , 2.3 and l, there are many possihil~tiesto jump o v ~ the r barrier. ('.g. ~~~~
~
~
~ - .
The 'harrier of A = 8,' i.e. the missing mass number A = 8 would seem to contradict the hwothesis that 4He is the main building block of the lighter ecments. Again the harrier can be oassed in several wavs: . . e.e. - with the aid of stable nuclei with A 6 or 7. However, the most common assumption is that 8Be was formed through the reaction
This means t h a t in Figure 2 we approach stability from the left along an isobar-line, since for each 8- decay N decreases and Z increases by one unit. In this case the first stahle isohar
=
Although SBe has a half-life of only 3 X 10-'6 s, the frequency with which readion (4) occurred led to a large enough steady state concentration of 8Be that it could react with neutrons to form 9Be or 4He to form I2C. The commonly accepted theory is that the formation of the light elements arises from 4He reactions. The basic reaction is
+
-
(5) $He ex e:lx Since in $He, N = Z, helium burning leads to the formation of nuclei with the same number of neutrons and of protons; i.e., N = Z = AIZ. This is reflected by the straight line for N = Z in Fieure 2 which is a plot of the relation between the number in stahle nuclei. The neutron nknberand the last stable nucleus with N = Z i s x a ; afkr that the coulombic repuls~onof the protons results in inrreaiing stability for 'neutron rich' nurlei ( N > 2). Thus if:He is captured by a nucleus with A > 20. an unstahle nucleus if formed. which decays along an isobar line, until a stahle ratio N I Z > 1 is obtained. This decay to a stahle isohar occurs through the emission of a positron (positive electron) or through capture of an orhital electron. For each decay step N increases and Z decreases by one unit, e.g. $X
-
a-,X
+ e+
(6)
The first stable isobar which is formed in such a decay sequence terminates the decay chain and would necessariiy be the isohar of highest Z if several stable isohars should exist for that particular A value. The consequence would he a higher ahundance of the stahle isobar of highest atomic number for the stahle isobars of the A chain. This is exactly the ahundance Dattern observed for almost all the isohar chains for A values -90, i.e., about up to the iron group of elements. The table shows all stableeven mass numhers from 36204; the mass numbers have been grouped according to whether the isohar of highest Z is the most ahundant (XH> XL) or is the least ahundant (XH< XL). The data in the table strongly support the hypothesis that nuclei up to about the iron group have been formed through helium capture. One problem we have not mentioned is that the $Henuclei must possess enough kinetic energy to overcome the coulomb repdsion as they approach the $X nuclei. However, up to A 4 0 , the helium capture reactions are exothermic and the energy released provides the necessary heating for a sufficient fraction of the :He to have the kinetic energy required. Most of the reactions leading to synthesis of elements of A d 70 are not as simple as that of eqn. (5).Protons and, to a smaller extent, neutrons may be released in the reactions. The concentration of protons is enough to allow in turn some degree of proton capture but this would still lead to the stahle isohar of hiehest Z heine the most abundant. Around i - 7 0 the tahie shows a 'cross-over' and for A > 70, the liahter isobars become more stable. The ex~lanationof elemental formation through helium- or proton-burning can no longer explain isobaric ahundances for elements heavier than the iron group. For these elements it is necessary to assume a different formation process. Neutnm rich nuclei pn,d;ced in fission of uranium, or by irradiating natural elements with neutrons in a nuclear reactor decay by 3--emission to stable elements $-'X t An
- ex -
$+,X
+ e-
(7)
Figure 2. Diagram of stable nuclei (the stabllily valley).Unstable (radioactive) nuclei (notshown) appear w, both sides of the stable nuclei (blackcircler) and decay along isobar iinas to the stable m s . Fmm llw pmorwich side lhis takes place through positron emission (P') 0, electron capture (EC)and from the neutron rich side through negatron (8-1 emission ( 10).
Comparison ot Isobar 01 Higheot Abundance for DMerenl Values of A
Most Abundant Isobar Highest Highest Z Z -Lowest Z
Abundancies
I%)
Lowest Z
.
.
.
192. 196. 198,204
Volume 54, Number 12, December 1977 / 743
rearhed is the one of lowest Z if several stable isobars exist for that value of A. This is thesituation ohserved for the isobars in the lower part of the table (A > 70). Therefore, i t is generally believed that the elements heavier than iron (Z = 26) have been synthesized through neutron capture reactions. Although an environment of high - neutron density exists in the first minutes after the "big-bang," the processes which we are discussing are believed to have occurred much later and are still occurring in stars. The elements heavier than iron are assumed t o have been formed through two different neutron capture processes. According to the slow (slow in relation to 8-decay processes) or s-process, a concentration of neutrons is produced by the bombardment of medium-heavy nuclides by high-energy He-particles in the interior of the red-giant stars; an example of such a neutron producing process is
-
:ANe + $He f$Mg + An (8) The neutrons are captured by other nuclei to form heavier species. The sequence of neutron capture has a long time scale (10%to lo5 yr per neutron capture) so beta decay processes interrupt i t to form different elements of higher Z. A typical s-process sequence might be
In the rapid or r-process, the conditions of pressure, temperature and photon emission may become so extreme in an old red giant star (i.e. one a t a much later age than when the sprocess began) that the iron group elements begin to undergo photodisintegration to release many lighter particles, e.g. 56Fe+y-:~i+6:He+4n (10) This photodisintegration leads t o a very rapid increase in energy-and in luminosity-of the star. Such events are observed as supernovae ex~losions.Durine this brief event. fantastically high neutron fluxes are p r e s e k Neutron capture occurs over such a short time scale that beta decay can take place only afterwards. For example, compare thi sequence below for the r-process with that in (10) for the s-process
-
:~Re+n-:PRu+n-:~u+n-:~Ru+n-:~Ru
+n
:i5Ru
+ n -:$Ru
(11)
Later Io3Ru,IMRu, 105Ru, etc. undergo beta decay to isotopes of Rh. The s-process should be able to form the elements up to bismuth. The r-process can form these as well as elements up
744 / Journal of Chemical Education
to the heaviest known. Some of these very heavy elements would be expected to fission and, in fact, the elemental composition of some stars indicates unexpectedly high abundancies for elements such as strontium, yttrium, and the rare earths, which are formed in highest yields in fission processes. Both the s - and r-processes contribute elements up tos3Bi. It is now believed that the cloud of elements. out of which our solar system condensed, had been synthekzed through rDrocesses in several different eenerations of stars. The last ;-process produced the elements up to uranium and even ~ l u t o n i u mwhich still exists in verv small traces on earth. ~ u r i n this g brief event, fantasticall; high neutron fluxes are present. We should note that we still must account for the abundance of the light stable isobars for A < 70. These nuclei are the heavier stable isotopes ( N > Z) of the lighter elements. I t is generally believed that the heavier fraction of elements lighter than iron were formed in a kind of equilibrium (the e-process) between protons, helium atoms, and heavier nuclei. Such an equilibrium is believed to persist for somr lrngth of time (-10' yrl in the interior of red-giant srars (temperature -lOq K I . This is a staee thrnurh " which all stars somevha~ larger than our sun (which is relatively small in comparison with most stars in our galaxy) must pass on their way from birth to their final disappearance from the sky, when all their energy resources are exhausted. In the table there are some exceptions to the abundancy rule. These occur a t Z and N numbers around 28.50.82. Nuclei with thew numbers of protons 1 8 1 neutrons are extremely stable (so-called ' m a ~ i cnumbers') and are thrrefore ex~erred to have greater than normal abundances (cf. also Fig. 2j. Such deviations were the first indications of a regular internal structure of the nucleus. Knowledge about this structure is required in order to give a more detailed explanation of the isotopic and elemental composition of the universe and how i t was formed.
-
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