Tables 61 1 (fo«u'«iW)AND614 roc 



NORMAL SERIES RELATIONS IN ATOMIC SPECTRA (continued) 



Terms of the same sort are distinguished by prefixing a lower case letter — a 3 D, b 3 D, etc. 

 Lines are represented as the difference of two terms — the lower one coming first, e.g., 

 a 3 D 2 - y 3 F 2 °. 



It is recommended that the lowest terms in the spectrum, and others of the same type, 

 multiplicity, and parity be lettered a, b, c — beginning with the lowest. Terms of the 

 opposite parity should be lettered z, y, x — beginning with the lowest. In this way all 

 resonance lines will be designated as a ( ) — z( )°. 



In most spectra there is a group of low metastable terms of the same parity as the 

 lowest term, separated by a considerable interval from higher terms of the same parity. 

 The letters a, b, c, d should be reserved for the low terms and the high terms should 

 begin uniformly with e. 



In a good many spectra, energy levels have been detected whose reality is proved by their 

 combinations, but which can not (at present) be fitted definitely into the scheme of 

 multiple terms. Such levels are to be denoted by Arabic numbers (beginning at the lowest 

 level). It can always be told whether such a level is odd or even, and the inner quantum 

 number can usually, and the multiplicity sometimes be assigned. The corresponding indices 

 are then added to the number, e.g., 4 23?. 



This notation suffices for the formal description of even the most complex spectra. A 

 further analysis giving the electron configurations responsible for each term should be 

 made so far as possible. Following Hund the individual electrons in an atom may be 

 defined by two quantum numbers, " azimuthal " l and " total " n. The latter is denoted by 

 a numerical prefix, the former by the letters s, p, d, ... as follows : 



TABLE 614. — Comparison Azimuthal and Bohr's Quantum Numbers 



Letter 



Azimuthal quantum number 1 



Bohr's quantum number k 



A 6d electron (for example) has n = 6, \ = 2. This is exactly equivalent to Bohr's nota- 

 tion 6 3 — the subscript by the value of k. Similarly 4p = 42, 5s = 51. (Note that nSj^l + !•) 



The spin of the electron s, being always \, need not be specified. 



The number of electrons of a given type in an atom is represented by an exponent, 

 e.g., 3d 6 - 



The quantities s and 1 are vectors, of the dimensions of angular momentum. By space 

 quantization they combine to give the vectors S, L, and J which define the spectroscopic 

 properties of the energy levels. The multiplicity R = S + 1. The details are discussed in 

 Hund's book. The total quantum numbers n are not vectors and do not give a resultant. 



A complete specification of an electron configuration would include all the inner elec- 

 trons — the normal state of iron, for example is is 2 2s 2 2p 6 3s 2 3p 6 3d 6 4s 2 . For ordinary spectro- 

 scopic purposes the complete shells may be omitted, reducing the foregoing to 3d e 4s 2 . For 

 brevity the total quantum numbers may be omitted when their values are the lowest which 

 the electron can have without belonging to already complete shells. For example, for 

 spectra from K I to Zn I, and Ca II to Ga II, these electrons are 4s, 4p, 3d, 4L The normal 

 state of Fe I is then represented by dV^Di and that of O II by s 2 p 34 Si*. 



Odd and even terms are those in which the sum of the l's for all electrons is odd or even ; 

 i.e., in which the number of p or f electrons together is odd or even. Completed shells always 

 give an even sum and may be disregarded. 



Smithsonian Tables 



