Why are some elements plentiful and others rare?

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aaron38

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Two days of googling hasn't led me to the answer. <br /><br />Specifically I was thinking about lead and gold. One is a common base metal, one is a rare precious metal.<br /><br />I've been trying to find out what is the process in the formation of the elements (in a supernova) that favor one element over another.<br /><br />I know that radioactive elements often decay to lead, so that would bias in favor of lead. Are there other processes in action to explain why rare-earth elemets are rare? Especially compared to other elements of near equal weights, such as lead and gold.
 
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rogerinnh

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I would suspect that it is determined at least partly by how difficult it is (how much engery it takes) to form atoms. The more protons, electrons, and neutrons it takes to form the atom the more difficult it is to force them together into a stable configuration. If you were to start out with a nice big bag of those elementary partilces you'd probably find it relatively easy to make hydrogen atoms (just an electron and a proton). It will take a bit more effort to produce a helium atom. And as you work your way up the periodic table of elements it gets more and more difficult to make the atoms. So, bottom line, lots of hydrogen, not so much gold.<br /><br />Now, as to why a given planet has more of one element than another element, and why some planets are huge balls of gas while others are mostly rock, that I don't have a good answer for.
 
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h9c2

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Rare earth elements are actually not very rare as far as abundance goes. The problem with these elements is that they do not often appear in high concentrations that would make it easy to mine them.
 
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yevaud

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Precisely correct. Nucleosynthesis is the word Aaron is looking for. The more massive the molecule, the more energy it requires to form. This is why simpler molecules are readily found, but the more complex grow rarer and rarer as you move up the periodic table.<br /><br />The answer to the second part is the process of planetary formation known as "differentiation." Simply, the planet begins as a largely molten object. The denser the substance, the more likely it is to settle deep into the interior and coversely, the lighter to the surface.<br /><br />Note Mars' reddish hue? It's due to the presence of numerous ferric oxides at or near the surface. A clear example of the fact that Mars - not a large planet by any means - differentiated faster than Earth did. There was less opportunity for heavier elements to settle deeper, and so a higher proportion than Earth is at or near the surface. <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>
 
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Saiph

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<blockquote><font class="small">In reply to:</font><hr /><p><br />Note Mars' reddish hue? It's due to the presence of numerous ferric oxides at or near the surface. A clear example of the fact that Mars - not a large planet by any means - differentiated faster than Earth did. There was less opportunity for heavier elements to settle deeper, and so a higher proportion than Earth is at or near the surface.<p><hr /></p></p></blockquote><br /><br />I'd restate the gist of this as: It's evidence that mars did not differentiate as thuroughly as earth, as it solidified faster. This more rapid solidification provided less oportunity for the heavier elements to settle (and thus the lower level of differentiation). It solidified faster, btw, because it is smaller, and has less heat from its formation. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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mikeemmert

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I read in Sky and Telescope in a sidebar article that the key to this is that Type II supernovae create mostly Californium 254, rather than creating a continuous spectrum of elements. The other posters on this thread have noted that more extreme conditions are required for the production of heavier elements. That's correct, and the extreme conditions are easily produced in the core of a Type II supernovae.<br /><br />Astronomer know Californium 254 is produced because it has a 55 day half-life, which matches the decay of the light curve in many supernovae. Probably other elements are also produced, but they would also be very heavy elements. If their half life is much shorter, they would decay before the envelope around the star was disrupted and so wouldn't be seen; if the half-life was much longer, not enough power (energy/time) would be produced to be detectable.<br /><br />These heavy elements then decay into lighter elements. Most decays by spontaneous fission, that is, fission that occurs without neutron bombardment. It is a radioactive decay mode that is significant in very heavy elements.<br /><br />A great deal of it, however, decays through alpha emission and produces lead, leading to the observed abundance of lead. Gold is not the end of the alpha decay chain. It is a product of spontaneous fission, and therefore is not nearly as common as lead. This can be said of a lot of elements between iron and lead.<br /><br />Tin is relatively common because it has a lot of stable isotopes and so there are a lot of ways spontaneous fission and subsequent beta decay can produce it.<br /><br />Unfortunately, my sources are all print, rather than internet. Try googling Californium 252.<br /><br />Oh yes, uranium. Uranium isotopes are quite stable compared to other actinides, so the alpha decay process is very much slowed down when it reaches uranium. That's why uranium is a fairly common element. It is more common than zinc or cadmium and is fourty times as common a
 
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yevaud

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Exactly so.<br /><br />I've often mulled over if the fact that it differentiated so fast compared to Earth might have something to do with it's relative lack of a Geomagnetic field.<br /><br />Just a little thought-experiment. <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>
 
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Saiph

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the main reason for it's rapid solidification (and thus lack of differentiation) is really a volume to surface area ratio. The planet stores heat in it's entire volume, and radiates it through the surface area. A rough analysis shows that a planet cools with a rate that is dependant on 1/r. I.e. , bigger planets cool slower. Even though they have a larger surface area to radiate it, they have far more heat to radiate. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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yevaud

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Yes, knew that. What I meant was that the agglomeration of material - ferrous and/or radioactive may have had some relevance in the relative lack of the planet's Geomagnetic field. Possibly there was a sufficiently low proportion of such materials at the core - due to rapid differention - that no "internal dynamo" really ever got started. <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>
 
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aaron38

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Thanks all. The lead on Californium 254 especially turned up quite a few papers on the formation of elements in stellar nuclear reactions and supernovae that pretty well answered my questions.
 
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Saiph

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ahh, gotcha. That could be a reason for the relatively weak magnetic field.<br /><br />Now, we need to figure out Venus's lack of one. I'm sure it's gotta do with it's bizzarre rotation (or basically lack thereof) but I wouldn't expect the core to be as stationary... <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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nacnud

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Well there are some big differences between Earth and Venus, I recon plate tectonics is a good place to start. Earth lost a lost of its lighter material when a Mars sized object hit it and formed the moon. I think this left the Earths crust relatively thin and allowed plates to form.<br /><br />Venus was never hit and therefore still has more lighter material and a thicker crust, perhaps this changed the rate of convection in the core and the corresponding magnetic field was smaller.<br />
 
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cpuguy1

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Many of the element's isotopes (above Au or Pb) will eventually decay to one of several stable elements close to Pb (and/or one of it's long lived half life isotopes).<br />You need to find an element nuclides or isotope decay chart, then try and go backward from gold to see what element isotope you need to start with.<br />(Maybe go backward from platinum - it's more per ounce.)<br /> <div class="Discussion_UserSignature"> <p><font size="2">The polarization of the alpha wave's primary phase of the modified parallel cross section of your brain will confirm you are not thinking up to your potential.  -  </font><font size="2">Professor West</font></p> </div>
 
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donpmitchell

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Nuclear synthesis is the big issue. Elements from hydrogen to iron are mostly generated by fusion, but the most stable nucei are favored. So for example, all even-numbered elements are more common than odd-number elements. <br /><br />The process starts out with hydrogen - /> helium. Next Carbon is produced in abundance, from the fusion of helium. Beryllium 8 is skipped, because it is extremely unstable.<br /><br />Elements beyond iron are produced by two processes, called "rapid" (super nova explosions) and "slow" (the gradual absorption of neutrons inside stars). Studies of cosmis ray abundance seems to indicate both processes are important.<br /><br />As for lead being more common than gold, it actually is not. Gold, Platnum, and Lead are all about equally abundant in the universe. In the crust of the Earth, chemical affinity is more important. Gold dissolves in iron, and most of the Earth's gold is concentrated in the core of the planet.<br />
 
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mikeemmert

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I have been reading up on the "s" (slow) process for making elements.<br /><br />It starts with nitrogen 14 fuzing with helium 4 to produce flourine 18. That then emits a postitron and a neutrino to produce the neutron. That then fuzes with helium 4 to produce neon 22, which fuzes with another helium nucleus to produce magnesium 25, liberating the neutron.<br /><br />There are other reactions that produce neutrons, but that's the important one.<br /><br />Neutrons have a half-life of 11 minutes, so the neutrons need to be asorbed in that time frame to produce heaveir elements, which is usually what happens. The starting material is plentiful iron, which is produced in abundance in Type Ia supernovae, the most common.<br /><br />So copper, for instance, is fairly common. But as the elements get heavier, their abundance decreases. As a result, zinc and cadmium are less common than uranium. Silver is 40 times less common than uranium. Platinum, palladium, and rhenium are rare, indeed. The rare-earth elements as a group are fairly common, but each element is fairly rare. They are found in concentrated ores because of their unusual chemistry, with each rare-earth element having a very similar chemistry to the other rare-earth elements. The beach sands of Florida (monazite) are rare-earth elements, and India has similar large deposits, as do other places. Here also you will find thorium and uranium, which are also rare-earth elements as far as their chemistry goes. Rare earth ores are indeed rare in Europe for some reason, which is why they are called rare-earth elements.<br /><br />This has public policy implications:<br /><br />1) If some dictator claims that a competing dictator is buying uranium from a third dictator, then the first dictator is lying. The dictators of South Africa, North Korea, India, and Pakistan all got their uranium from their own countries. This does not apply _only_ to current or recent dictators, but also to future dictators.<br /><br />2) The
 
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