1 Familiar ways of classifying the elements IA IIA IIIB IVB VB VIB VIIB VIIIB IB IIB IIIA IV A V A VIA VIIA Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb T e I Xe Y Rb SrHfT a W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn La Cs Ba B C N O F Ne Li Be Al Si P S Cl ArNa Mg Ti V CrMn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Sc K Ca Ac Fr Ra 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 VIIIA 58 59 60 61 62 63 64 65 66 67 68 69 70 71 90 91 92 93 94 95 96 97 98 99 100 101 102 103 p e r i o d H 1 2 3 4 5 6 7 1 He 2 metals non-metals semi-conductors (semi-metals; metalloids) IA IIA IIIB IVB VB VIB VIIB VIIIB IB IIB IIIA IV A V A VIA VIIA VIIIA Zr Nb Mo T c Ru Rh Pd Ag Cd In Sn Sb T e I Xe Y Rb SrHfT a W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Cs Ba B C N O F Ne Li Be Al Si P S Cl ArNa Mg Ti V CrMn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Sc K Ca Fr Ra 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 2 8 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 4 5 46 47 48 49 50 51 52 53 54 55 56 57- 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89- 103 He H 1 2 2 3 4 5 6 7 1 transition elements Alternatively ... COSMIC ABUNDANCE of the ELEMENTSand NUCLEOSYNTHESIS February 3, 2005
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These geochemical tendencies largelydetermine where in the Earth many
elements are sequestered – e.g.,siderophile elements are concentratedin the core; volatile elements werepartially lost from the Earth during itshot early history.
similarilty in composition of chondrites and solar atmosphere...
Note: This diagram is a useful illustration, but bear in mind that:
1) it is a log-log plot ( and just about anything can appear to be linear on such a plot!)
2) the chondrites represented are actually metamorphic rocks, so their compositionmay not be "primitive" in all respects
O
MgFe
SNa Al
CaNi
Cr
MnP
CoK
Ti
ZnCu
GeScSr
BRb
BaY
Pb
Li
CeLa
BePr
ThTm
log relative abundance in C1 chondrites (Si = 10 )610
-2 0 2 4 6 8
l o g
r e l a t i v e a b u n d a n c e
i n s o l a r a t m o s p h e r e
( S i = 1 0
)
1 0
6
-2
0
2
4
6
8
*
* Chondrites are the most "primitive" of meteorites -- i.e., ones we believe representthe original, overall composition of the solar system. Five distinct types of chondritesare recognized.
"Burning" of hydrogen and helium in first-generation stars
First-generation stars are ones that have come together from an"original" hydrogen-helium cloud – that is, they do not represent "re-processed" stellar material from a previous supernova explosion (theSun is actually a second-generation star ). In first-generation stars,hydrogen "burning" to produce helium occurs by these reactions:
H11 H
11
+ H21 + energy
H21 H
11+ He
32 + energy
He32He
32
+ He42 H
11H
11
+ energy++
The elements as we know them are created in the interiors of stars...
On the main sequence, helium is the main product of hydrogenburning. A gravitationally stable core of helium is produced in the star,and hydrogen burning continues on the surface of the core. As H-burning migrates outward, the luminosity of the star increases slightly:
If the star is massive enough to achieve the required temperature anddensity in the core, is initiated and heavier elementscan be synthesized
This process is exceedingly inefficient because is unstable and-16
decays with a half-life of only 10 s. What probably happens is thatanother helium nucleus is immediately absorbed to make a carbonnucleus:
or
helium burning
He42+He
42 Be
84
Be84
+Be84 He
42 C
126
+ He42 C
126He
42He
42
+
hydrogen burning
Note: These helium nuclei are actually positively-charged alphaparticles and repel each other strongly (remember Mr. Coulomb?).Extreme temperatures and pressures are required to get them tofuse: He burning can occur only in stars having masses of 80% ormore of our Sun. This "triple-alpha" process is the key tomaking heavier elements...
nuclear fuel exhausted; star collapses to white dwarf
5
helium burning
He burning sustains red giants
for a few 10s of Ma at most
Somewhat heavier elements...
If a red giant is sufficiently massive, successively heavier elements can besynthesized by addition of alpha particles (He nuclei) to carbon...
56 56This "alpha-process" can continue up to Ni (which decays to Fe), butelements heavier than Fe cannot be made by this process because the
repulsion between large, positively-charged nuclei and particles is too strong.
Note that the nuclei forming by this process are all even-Z. Smallerabundances of odd-Z nuclei are produced by reactions among the fusionproducts, such as:
+ He42C
126
16O8
16O
8
+ He4
2
20Ne
10etc.
+H
1
1C
12
6
23
Na11C
12
6 +
and16
O8+ 31
P1516
O8 H11+
This creates further possibilities, e.g...
and
+ H11C
126
13N7 + then
13N7
13C6
++
+
16O8
+ +13C6 He
42
nThis type of reaction iscrucial because neutrons area product! (more soon)
That's about as much as we need to know about first-generation stars. The
key thing to remember is that when these explode, they contribute elementsheavier than H and He to the interstellar gas, so the next generation of stars canbegin with a different, more "versatile" fuel. Subsequent stars can burn hydrogenin the CNO cycle, in which hydrogen nuclei are added to carbon to produce firstnitrogen and then oxygen. This mode of H burning requires less extremeconditions than the proton-proton fusion reactions on p. 6. The Sun is nowburning H in the CNO cycle:
+ H11C
126
13N7
13N7
decaysC
136
C136 + H
11
14N7
15O8
decays 15N7
14N7 + H
11
15O8
He42
+15N7 + H
11 C
126
4 12end result: 4 protons fused to make He; C "released" for future use
Synthesis of heavy nuclei by neutron capture:the and in second-generation starss- r-processes
Elements heavier than Fe: NEUTRON CAPTURE
If there is a source of neutrons, the following type of reaction can occur
If a nucleus absorbs too many neutrons, it will eventually become too neutron-rich to be stable and decay by beta decay. Through neutron-capture reactions, itis possible to work up through most of the periodic table.
We recognize two distinct types of neutron-capture processes, which differ interms of the neutron flux required:
s-process: moderate neutron fluxes in the late red-giant stage
An example of neutron capture followed by beta decay (as in the s-process)...
Neutron activation is an analytical technique used extensively in the bio-,geo- and materials sciences for measurement of trace concentrations (e.g.1-100 ppm) of elements in a wide variety of materials. As the name suggests,it involves the use of a neutron flux in a research reactor to “activate” theelement of interest. This really means that the element (nucleus) of interestis “transmuted” -- by absorbing a neutron -- into a heavier nucleus that isradioactive. This “activated” nucleus is then detected by recording thegamma ray emitted when it disintegrates by beta decay.
The sequence just described (neutron absorption followed by beta-decay)is analogous to a step along the s-process path (see class hand-out). Theonly difference in the case of neutron activation is that the source of neutrons
is a man-made reactor (and the target nuclei are different).
Here’s an example: Analysis of SiO glass for sodium impurities...2
mostly SiO with2
a few Na atomsneutrons
23 24 23 24Na is “activated” by conversion to Na: Na + N Na
24 ( Na is radioactive)
Step 1 Place glass sample in reactor
24Step 2 Detect Na decay events by counting rays produced by the reaction