Atomic States and Term Symbols

ATOMIC STATES AND TERM SYMBOLS OF FREE IONS... 

The progression of sections leads the reader from the principles of quantum mechanics and several model problems which illustrate these principles and relate to chemical phenomena, through atomic and molecular orbitals, N-electron configurations, states, and term symbols, vibrational and rotational energy levels, photon-induced transitions among various levels, and eventually to computational techniques for treating chemical bonding and reactivity. 

A.12-1. Spectroscopic states and term symbols for many-electron atoms or... 

Give the electronic configurations and term symbols of the first excited electronic states of the atoms up to Z = 10. 

The first excited state of the molecule, c N , is built from normal atoms, and has the term symbol 2n. It lies 1.78 v. e. above the normal state. 

In a manner similar to that by which the atomic states were designated as s, p, d, or /, the letters S, P, D, and F correspond to the values of 0, 1, 2, and 3, respectively, for the angular momentum vector, L. After the values of the vectors L, S, and / have been determined, the overall angular momentum is described by a symbol known as a term symbol or spectroscopic state. This symbol is constructed as Ps+1)Lj where the appropriate letter is used for the L value as listed earlier, and the quantity (2S + 1) is known as the multiplicity. For one unpaired electron, (2S + 1) = 2, and a multiplicity of 2 gives rise to a doublet. For two unpaired electrons, the multiplicity is 3, and the state is called a triplet state. 

The term symbol summarizes the properties of any state and also permits a concise representation of spectral transitions. It consists of an upper case letter (S, P, D. ..) to represent the net orbital angular momentum (L) and a number written as a superscript on the upper left to indicate spin multiplicity (i. e. the number of possible orientations of total spin of the atom). L is zero for Fe " (no angular momentum) and 2 for Fe ". The spin multiplicity is defined as (2S -i- 1) S = 5/2 and 2 for Fe " and Fe ", respectively. The ground state term symbol for Fe " is, therefore, 85 2 and for Fe " it is 04. The subscript on the right is the value J. 

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