Hund's rules: Difference between revisions

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imported>Paul Wormer
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* The ground state carbon atom, (1''s'')<sup>2</sup>(2''s'')<sup>2</sup>(2''p'')<sup>2</sup>, gives by [[Russell-Saunders coupling]] a set of energy levels labeled by [[term symbol]]s. Hund's rules predict the following order of the energies:
* The ground state carbon atom, (1''s'')<sup>2</sup>(2''s'')<sup>2</sup>(2''p'')<sup>2</sup>, gives by [[Russell-Saunders coupling]] a set of energy levels labeled by [[term symbol]]s. Hund's rules predict the following order of the energies:
::<math>
::<math>
^3P_{0} < ^3P_{1} < ^3P_{2} < ^1D_{2} < ^1P_{2}.
^3P_{0} < ^3P_{1} < ^3P_{2} < ^1D_{2} < ^1S_{0}.
</math>
</math>
* The ground state oxygen atom, (1''s'')<sup>2</sup>(2''s'')<sup>2</sup>(2''p'')<sup>4</sup>, (a two-hole state) gives by Russell-Saunders coupling a set of energy levels labeled by term symbols. Hund's rules predict the following order of the energies:
* The ground state oxygen atom, (1''s'')<sup>2</sup>(2''s'')<sup>2</sup>(2''p'')<sup>4</sup>, (a two-hole state) gives by Russell-Saunders coupling a set of energy levels labeled by term symbols. Hund's rules predict the following order of the energies:
::<math>
::<math>
^3P_{2} < ^3P_{1} < ^3P_{0} < ^1D_{2} < ^1P_{2}.
^3P_{2} < ^3P_{1} < ^3P_{0} < ^1D_{2} < ^1S_{0}.
</math>
</math>
==References==
==References==

Revision as of 03:36, 12 January 2008

In atomic spectroscopy, Hund's rules predict the order of atomic energy levels with quantum numbers L, S and J. The rules are called after Friedrich Hund who formulated them in 1925.[1]

A group of atomic energy levels, obtained by Russell-Saunders coupling, is concisely indicated by a term symbol. A term (also known as multiplet) is a set of simultaneous eigenfunctions of L2 (total orbital angular momentum squared) and S2 (total spin angular momentum squared) with quantum numbers L and S, respectively. If there is no spin-orbit coupling, the functions of one term are degenerate (have the same energy). If there is (weak) spin-orbit coupling it is useful to diagonalize the matrix of the corresponding operator within the LS basis in the spirit of first-order perturbation theory. This introduces the new quantum number J, with |L-S| ≤ JL+S, that labels a 2(J+1)-dimensional energy level.

Hund's rules are:[2]

  1. Of the Russell-Saunders states arising from a given electronic configuration those with the largest spin quantum number S lie lowest, those with the next largest next, and so on; in other words, the states with largest spin multiplicity are the most stable.
  2. Of the group of terms with a given value of S, that with the largest value of L lies lowest.
  3. Of the states with given values of S and L in an electronic configuration consisting of less than half the electrons in a closed subshell, the state with the smallest value of J is usually the most stable, and for a configuration consisting of more than half the electrons in a closed subshell the state with largest J is the most stable.

The levels of the second sort, largest J most stable, can be seen as arising from holes in a closed subshell.

Examples:

  • The ground state carbon atom, (1s)2(2s)2(2p)2, gives by Russell-Saunders coupling a set of energy levels labeled by term symbols. Hund's rules predict the following order of the energies:
  • The ground state oxygen atom, (1s)2(2s)2(2p)4, (a two-hole state) gives by Russell-Saunders coupling a set of energy levels labeled by term symbols. Hund's rules predict the following order of the energies:

References

  1. F. Hund, Zur Deutung verwickelter Spektren, insbesondere der Elemente Scandium bis Nickel. [On the interpretation of complicated spectra, in particular the elements scandium through nickel]. Zeitschrift für Physik, vol. 33, pp. 345-371 (1925).
  2. L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithaca, 3rd edition (1960)