Helium Energy LevelsThe electron energy levels for a helium atom demonstrate a number of features of multi-electron atoms. One electron is presumed to be in the ground state, the 1s state. An electron in an upper state can have spin antiparallel to the ground state electron (S=0, singlet state, parahelium) or parallel to the ground state electron (S=1, triplet state, orthohelium).
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Index Atomic Structure Concepts | |||||||
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Orthohelium and Parahelium Energy LevelsIn the helium energy level diagram, one electron is presumed to be in the ground state of a helium atom, the 1s state. An electron in an upper state can have spin antiparallel to the ground state electron (S=0, singlet state, parahelium) or parallel to the ground state electron (S=1, triplet state, orthohelium). It is observed that the orthohelium states are lower in energy than the parahelium states. The explanation for this is:
This effect is sometimes called the "spin-spin interaction" and is addressed by Hund's Rule #1 . It is part of the understanding of the ordering of energy levels in multi-electron atoms.
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Index Reference Rohlf Ch 9 | ||
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Helium Energy LevelsThe helium ground state consists of two identical 1s electrons. The energy required to remove one of them is the highest ionization energy of any atom in the periodic table: 24.6 electron volts. The energy required to remove the second electron is 54.4 eV, as would be expected by modeling it after the hydrogen energy levels. The He+ ion is just like a hydrogen atom with two units of charge in the nucleus. Since the hydrogenic energy levels depend upon the square of the nuclear charge, the energy of the remaining helium electron should be just 4x(-13.6 eV) = -54.4 eV as observed. The fact that the second electron is less tightly bound can be interpreted as a shielding effect; the other electron partly shields the second electron from the full charge of the nucleus. Its energy can be used to model the effective shielding as follows. Another way to view the energy is to say that the repulsion of the electrons contributes a positive potential energy which partially offsets the negative potential energy contributed by the attractive electric force of the nuclear charge. The description of any electron in a multi-electron atom must find a way to characterize the effect of the other electrons on the energy.
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