Simple Atomic Mass Unit question

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  1. little pixie's Avatar
    1 06-05-2012  16:00
    • Exalted Member
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    Simple Atomic Mass Unit question
    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

    The answer is given as

    n + 235U → 93Rb + 141Cs + 2n

    LHS 235.0439 + 1.0087
    RHS 92.921712 + 140.91949 + 2.10087

    Difference is 0.1943u = 181 MeV

    In the answer, how is it known that 2n should be added? Why 2?
    Last edited by little pixie; 06-05-2012 at 17:06.
  2. ASaunders's Avatar
    2 06-05-2012  17:31
    • Junior Member
    • Posts: 54
    Re: Simple Atomic Mass Unit question
    The nucleon number (mass number) has to be conserved. On the left hand side of the equation the total nucleon number is 236. The isotopes of Rubidium and Caesium have a total nucleon number of 234, hence 2 neutrons.
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  3. little pixie's Avatar
    3 06-05-2012  17:41
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    • Posts: 392
    Re: Simple Atomic Mass Unit question
    That's great! Thanks very much for that ASaunders!
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