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Homework Help: Binding energy of C12

  1. Feb 17, 2014 #1
    1. The problem statement, all variables and given/known data

    Given that the mass of 1 H is 1.00794 amu, the mass of a neutron is 1.00865 amu and
    that the energy equivalent of 1 atomic mass unit (AMU) is 931.5 MeV, what is the
    binding energy of 12C (atomic mass = 12.00000 amu)?

    2. Relevant equations

    (σ) mass decrement=(W) mass of constituent particles (aka for helium 4 its 4p+4n+4e) - (M) actual mass of atom

    3. The attempt at a solution

    I want to find the binding energy of carbon 12. So it's σ=W-12. W is 6p+6n+6e. I know the mass of n. But I do not know the mass of p and e.

    I am told that 1H=1.00794 amu. So 1.00794=1p+1e-σ. I don't see how this helps me get values for p and e. Help?
  2. jcsd
  3. Feb 17, 2014 #2


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    What is the Hydrogen atom ?

  4. Feb 17, 2014 #3
    A hydrogen atom is one proton and one electron if I remember correctly. Am I supposed to derive the mass of p and e from this?
  5. Feb 17, 2014 #4


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    You need only the mass of 6p + 6e=6(p+e). The proton and electron do not merge in the hydrogen atom, they are relatively far away, you can take the mass of the hydrogen atom equal to the mass of the proton + mass of the electron.

    Last edited: Feb 17, 2014
  6. Feb 17, 2014 #5
    Ooh I see. The lack of neutron means I can neglect binding energies. That makes much more sense thank you.
  7. Feb 18, 2014 #6
    Hmm, I just checked the given values of the mass of a proton and electron from the book I have (where the question is from) and I notice that the given mass of the 1 H and what can be added from the following data does not add up.

    proton: 1.007276 u
    electron: 0.00054858 u
    p+e=1.00782 which does not = 1.00794

  8. Feb 18, 2014 #7


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    There is some confusion between the atomic mass and the relative atomic mass The atomic mass or relative isotopic mass refers to the mass of a single particle, and is fundamentally different from the quantities elemental atomic weight (also called "relative atomic mass") and standard atomic weight, both of which refer to averages (mathematical means) of naturally-occurring atomic mass values for samples of elements

    The hydrogen element occurs as mixture of hydrogen, deuterium and tritium isotopes, its relative atomic mass is 1.00794 ± 0.00001 u.
    The hydrogen isotope, which consist of a single proton and a single electron, has atomic mass of 1.00782505 u.

    So the problem gave a wrong value ...the relative atomic mass of the element instead of the atomic mass of the hydrogen isotope.

    Last edited: Feb 18, 2014
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