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Space that atoms occupy?

  1. Dec 22, 2012 #1
    When two hydrogen atoms that each occupied an estimated amount of space combine in a nuclear fusion, that new atom(helium) actually occupies an estimated less amount of space than even one of the hydrogen atoms....Where does that space go if space actually has weight or energy(Which is what lawrence Krauss stated)

    It's hard for to imagine this until I heard the analogy of a fly in the middle of the stadium would be a blown up scale of an atom. When I imagine smashing 2 stadiums together and the result is a smaller stadium, surely that space one of the stadiums occupied has to go somewhere. Is there anyway to calculate this either at the atomic level or scaled up stadium level.

    Thank you

    Adam C.D.
    Last edited: Dec 22, 2012
  2. jcsd
  3. Dec 22, 2012 #2


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    The analogy of two stadiums to two atoms is a bit awkward and not appropriate in the context of fusion of nuclei.

    When two protons fuse, the form deuterium, an isotope of hydrogen, and the deuterium atom is about the size of a hydrogen (protium) atom. In the process, a positron is given off, and that anihilates one of the electrons from one of the original hydrogen atoms. Acutually, in fusion plasmas, there are very few atoms, if the temperature high. Plasma implies that atoms have been fully ionized, so the plasma consists of nuclei and free electrons. There may be some level of recombination, but collisions with nuclei and electrons will readily ionize an atom.


    An atom after all is only a nuclei with atomic electrons surronding it. All atoms are similar in size with a variation of one order of magnitude from atomic radii from 31 pm (He) to 298 pm (Cs) with Fr being larger, probably on the order of 340 pm.

    Note that He nucleus (equivalent to an alpha particle) has a charge of +2, so it 'pulls' or 'attracts' its atomic electrons more strongly than a hydrogen nucleus.

    In fusion, the separate nuclei combine to for a single nucleus, although in most fusion reactions, generally one large nucleus (by mass) will form with a smaller one as the other product; for example, d+t = α + n. The fusion process releases energy to the reactants as the nucleons in the nucleus reconfigure to a more tightly bound configuration. This is related to the binding energy, the energy that one must put in to separate or break apart a nucleus.
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