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About isotopes

  1. Mar 30, 2014 #1
    I'm in year 10 and I have been studying physics and chemistry. When I learned isotopes, I notice that for isotopes, the number of neutron is always higher than that of proton. Is it possible that there are isotopes that the number of neutron is lower than that of proton? If no, is that because repulsion will take place if the number of neutron is lower than that of proton thus making the nucleus easy to spilt up? If I said something wrong or non-sense, please forgive me.
  2. jcsd
  3. Mar 30, 2014 #2


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    Yes. H1 has one proton and zero neutrons. :smile:
  4. Mar 30, 2014 #3


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    Also He3, two protons and one neutron. Notice that it is stable in contrast to H3, one proton and two neutrons.

    There are others, but they tend to be unstable.
  5. Mar 30, 2014 #4


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    One can look at nuclides (isotopes) and their properties here -

    He-3 is the only stable isotope with more protons than neutrons. Essentially, stability requires the same number or greater number of neutrons than protons, with some limits. When the number of protons exceeds the neutrons, then electron capture or positron emission generally is the decay mode.
  6. Mar 31, 2014 #5
    And the other is proton.
    Look at the simple cases of 1 excess protons:
    Proton: Stable. Neutron undergoes beta decay, releasing 782 keV and half-life 10 minutes
    He-3: Stable. Triton undergoes beta decay... but releases only 18 keV, with half-life 12 years. Another consequence of neutron having bigger binding energy in T that a proton has in He-3 is that He-3 easily captures slow neutrons, expelling a proton and releasing energy.
    Li-5: no set of 5 nucleons is bound.
    Be-7: bound but unstable. Unlike n (beta decay 782 keV) and t (beta decay 18 keV), Li-7 is stable to beta decay, instead Be-7 captures an electron releasing 862 keV, with half-life 53 days. Note that Be-7 cannot emit a positron, because electron capture releases just 862 keV, and creation of electron-positron pair would take 1022 keV
    B-9: unbound (unlike Be-9, which is bound and stable)
    C-11: would release 1982 keV on electron capture, therefore can also emit a positron with 960 keV left over, and does so, with half-live 20 minutes.
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