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Why does an Alpha Particle have mass 4.00151u? (As opposed to less than 4u) watch

    • Thread Starter

    If I understand correctly, 1u = 1/12th the mass of a carbon-12 atom. (6p + 6n + 6e). So for a carbon 12 atom it has mass u. And carbon 12 nucleus has mass 11.9967u (which would be expected since there aren't any electrons).

    However if I apply the same logic to a helium atom. Then surely for an alpha particle (helium nucleus) it should be less than for the atom. So does this mean helium atom has mass greater than 4.00151? I was under the idea that elements with perfect ration of p : n : e would have mass in u equal to their mass number (mr value). So helium atom was 4u?

    A helium atom (plus electrons) has a mass of 4.00260u
    (Without the electrons you get 4.0015u)

    It is made up of two neutrons (each of mass 1.00867) and two hydogen atoms (proton+electron) each of mass 1.00780u
    (proton=1.00728u, electron=0.00055u)
    This gives a total for the particles of 4.03300u

    The difference accounts for the binding energy.

    The mass of the nucleus is always less than the sum of the mass of the individual nucleons.

    Yes. There are particular stable configurations of nucleons.
    For smaller nuclei, equal numbers of protons and neutrons is such a stable configuation.

    yeh, He isn't exactly 4u
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