Helium atoms in singlet states (antiparallel spins) constitute paraheliumand
those in triplet states (parallel spins) constitute orthohelium.An orthohelium atom
can lose excitation energy in a collision and become one of parahelium, while a
parahelium atom can gain excitation energy in a collision and become one of or-
thohelium. Ordinary liquid or gaseous helium is therefore a mixture of both. The
lowest triplet states are metastable because, in the absence of collisions, an atom in
one of them can retain its excitation energy for a relatively long time (a second or
more) before radiating.
2 Another obvious peculiarity in Fig. 7.24 is the absence of the 1^3 Sstate in helium.
The lowest triplet state is 2^3 S,although the lowest singlet state is 1^1 S.The 1^3 Sstate is
missing because of the exclusion principle, since in this state the two electrons would
have parallel spins and therefore identical sets of quantum numbers.
3 The energy difference between the ground state and the lowest excited state in
helium is relatively large. This reflects the tight binding of closed-shell electrons dis-
cussed earlier in this chapter. The ionization energy of helium—the work that must
be done to remove an electron from a helium atom—is 24.6 eV, the highest of any
element.n
∞Excitation
energy, eV5010152024.6
4
321Singlet states
(parahelium)Triplet states
(orthohelium)(^1) S (^1) P (^1) D (^1) F (^3) S (^3) P (^3) D (^3) F
262 Appendix to Chapter 7
Figure 7.24Energy-level diagram for helium showing the division into singlet (parahelium) and triplet
(orthohelium) states. There is no 1^3 Sstate because the exclusion principle prohibits two electrons with
parallel spins in the same state.
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