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Why doesn't the most common form of the hydrogen atom have a neutron?

  1. Jun 29, 2011 #1
    Most common isotopes of He has 2 neutrons, Li has 3 neutrons and so on right, until Z increases to higher numbers and we get to elements like iron, where the nucleus doesn't have equal numbers of protons and neutrons anymore. But why isn't the number of protons and neutrons equal in the most common form of Hydrogen, which is the most basic atom there is?

    Thanks.
     
    Last edited: Jun 29, 2011
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  3. Jun 29, 2011 #2

    Bill_K

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    Deuterium was produced in the Big Bang. And it's quite easy to make, but apparently the trick is keeping your deuterium once you have produced it. Helium-4 is so doggone stable that conditions in which deuterium can form are also conditions in which deuterium fuses to Helium-4.
     
  4. Jun 29, 2011 #3

    Vanadium 50

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    The relative abundance of deuterium in the early universe was at its highest still only about 1%.

    The amount of deuterium you can make is limited by the number of neutrons you have, and once you get neutrons, as Bill said, they tend to get locked up into heavier elements like Helium.
     
  5. Jul 15, 2011 #4

    Deuterium is also the most weakly bound compound nucleus and breaks apart easily inside stellar cores.
     
  6. Jul 15, 2011 #5
    Then the question arises "why is it so weakly bound?"Does this has something to do with charge/mass ratio of the nucleus?
     
  7. Jul 15, 2011 #6

    Drakkith

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    I think so. It is due to there only being 2 particles to attract each other. All other nuclei have 3+ nucleons or are unstable.
     
  8. Jul 15, 2011 #7

    bcrowell

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    In response to the OP's question, I think the basic answer is that the abundance of light nuclei is determined by big-bang nucleosynthesis, which occurred at high temperatures. At those temperatures, there was a tendency for deuterium to break up. In comparison, helium is doubly magic http://en.wikipedia.org/wiki/Magic_number_(physics) , so it's extremely stable, and any helium that formed in big bang nucleosynthesis was likely to hold together.

    Another point to note is that although there are reactions in stars that destroy deuterium, there are none that create it. Therefore the abundance of deuterium in our universe is like a ticking clock that started counting down after the big bang. The fact that its abundance isn't zero is actually very strong evidence for the finite age of the universe. If the universe was infinitely old, then there would be no deuterium left.

    No, electromagnetic interactions are irrelevant to the stability of light nuclei. When you have a set of Z protons interacting, the number of interactions is Z(Z-1)/2, i.e., it basically grows like Z^2. In a heavy nucleus like uranium, with Z=92, electrical repulsion is a major player, but not for small Z. For Z=1 the Coulomb energy vanishes, which would tend to make the nucleus *more* stable.

    For light nuclei, the most stable ratio of Z/A is 1/2. This isn't because the protons are charged. It's because of the Pauli exclusion principle, which favors equal numbers of protons and neutrons. Again, this should actually favor the stability of deuterium.

    A couple of generic reasons why we should expect deuterium to be unstable:

    (1) Nuclei are most bound in the region around iron. Nuclei lighter and heavier than iron are less bound, which is why you can generate energy by fusion of light nuclei and fission of heavy nuclei.

    (2) Odd-odd nuclei are always unstable compared to odd and even-even nuclei.

    What's actually quite surprising is that deuterium is *stable* with respect to beta decay. Almost all odd-odd nuclei are unstable.
     
  9. Jul 17, 2011 #8
    I'd argue its largely related to the condition that created the other nucleons.

    In stars there are lots of protons and neutrons, and the gravity is strong enough to bring them close together, where the nuclear force is notable.

    Outside of stars, there are many more protons than neutrons if for no other reason, due to substantially different half lives. Plus there are very few forces bringing the two together. I.e, if a nucleon is "made" in open space, its a very rare event. Usually protons just carry on their own way.

    So essentially it isn't that hydrogen is special, its that all of the other nuclei are.

    Incidentally what binds the deuteron is the nuclear spins. They align in the singlet state, which is the only bound state of the deuteron.
     
  10. Jul 17, 2011 #9

    bcrowell

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    This is all wrong. It sounds like you're imagining deuterium being produced in the present-day universe. The deuterium that presently exists in the universe was formed in big-bang nucleosynthesis: http://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis
     
  11. Jul 17, 2011 #10
    The point I was trying to make is that protons and neutrons don't just come together and form deuterium for no reason, i.e. free protons are the norm, and bound states (such as deuterium, and heavier nucleons) are the exception.
     
  12. Jul 17, 2011 #11

    bcrowell

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    Big-bang nucleosynthesis happened in the early universe, which was very dense. There is also no Coulomb barrier between a neutron and a proton. So I'm serious, what you said was just completely wrong. If you want to learn something about big-bang nucleosynthesis, a very cool popular-level book is The First Three Minutes, by Steven Weinberg.
     
  13. Jul 18, 2011 #12

    Drakkith

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    There are few, if any, free neutrons in the cores of stars. Neither the triple alpha process, the proton-proton chain, nor the CNO cycle release free neutrons. The neutrons that are made are a result of beta decay and are completely bound to the nuclei in which they form.

    I believe it is the strong force that binds both nucleons together in a deuteron. The alignment of the spin is simply the lowest energy level between the available options.
     
  14. Jul 18, 2011 #13
    You are right that is the better picture. I was thinking in terms of post big bang. But as you say, everything was cooking together at the start. Incidentally do you know what the binding energy of the deuteron would have to be for the universe to be deuteron rich and hydrogen poor?
     
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