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Homework Help: Simple Atomic Mass Unit question

  1. May 6, 2012 #1
    1. The problem statement, all variables and given/known data
    The uranium isotope 235U captures a neutron and undergoes fission to produce 93Rb and 141Cs. Calculate the energy released in this process.

    The nuclear masses of the relevant isotopes are
    235U 235.0439u,
    93Rb 92.9217u,
    141Cs 140.9195u

    2. Relevant equations

    3. The attempt at a solution

    The answer is given as

    I realise the quantity of 1.0087 is the mass of neutron divided by one unified atomic mass unit. So that's (1.67493 x 10^-27) / (1.66 x 10^-27).
    Also, the 2.10087 number is simply double this.

    How have the 235.0439, 92.921712 & 140.91949 values been calculated? I know it's to do with E=Mc^2

    Thank you.
  2. jcsd
  3. May 6, 2012 #2
    It all has to do with the mass difference, (mass before) - (mass after) = (released) or (consumed energy). The values for the nuclei´s masses has to be taken from a table. There is no way of calculating the mass of a nuclei, it has to be measured.
  4. May 6, 2012 #3
    Can it not be calculated from the part of the question which states;

  5. May 6, 2012 #4
    The difference in mass is what is relevant, ((Mass before)-(Mass after)).*c^2 = E. To calculate this you also need the mass of the neutron. The mass of the neutron can not be calculated and needs to be taken from a table. Also in order to get the mass in kilograms for the other nuclei you simply multiply it with the atomic mass unit, just as with the neutron.
  6. May 6, 2012 #5
    In the answer, how is it known that 2n should be added? Why 2?
  7. May 6, 2012 #6


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    Staff: Mentor

    Take a look at the reaction - how many neutrons are produced?
  8. May 6, 2012 #7
    Is it because the total nucleon number on both sides must equal. Since LHS=236 RHS can only equal this by adding 2n.
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