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Homework Help: Ionisation levels in hydrogen and helium

  1. Oct 11, 2015 #1
    1. The problem statement, all variables and given/known data

    a) Explain qualitatively why the energy required to remove one electron from the ground state of the helium atom is much larger than that in hydrogen, while for other states the hydrogenic value is quite a good approximation, e.g. the 1s2s levels in helium have energies very similar to the 2s energy in hydrogen.

    b) The 1s2s configuration in helium is split into two levels. Explain briefly the physical origin of the splitting, and how it comes about that these different energies should be associated with different relative orientations of the intrinsic spins of the two electrons (so that one of the levels is called a “singlet”, and the other a “triplet”).

    2. Relevant equations


    3. The attempt at a solution

    a) The ground state of helium has direct exposure to a greater charged nucleus than hydrogen so the ionisation energy is greater. The other levels of helium are shielded by previous levels' electrons.

    b) no idea.

  2. jcsd
  3. Oct 11, 2015 #2


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    Science Advisor
    Homework Helper

    a) I agree with you.

    b) "Singlet" and "triplet", these are your keywords. Splitting occurs because there is the total wavefunction must obey Pauli principle, i.e. it must be antisymmetric.
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