The Periodic System. Closed Shells

We have already frequently referred to the periodic system, or table (see Table I, p. 35 fig. 14 shows a somewhat different form). The periodic table is an arrangement of the elements in a scheme 

We begin with the simplest element—hydrogen (Z=l). Its electron in the ground state is in the lowest orbit, and has therefore for its 

If we now add a third electron, there is no place for it in the K shell, and so it must settle in the L shell. Let us count the number of places in the L shell. First, as in the K shell, there are 2 electrons with I == 0, distinguishable only by their spin components next, come 6 places for electrons with Z == 1, since of course can take the three values —1, 0, 1, and for each of these there are two possible settings for the spin. There are therefore 8 places altogether in the L shell, and these are arranged (Stoner, 1924) in two sub-shells with 2 and 6 electrons. When the atomic number is steadily raised, the L shell becomes gradually filled the atoms concerned are Li, Be, B, C, N, 0, F, Ne, With Ne the L shell is completed a new electron must, settle in the M shell (Table IV, pp. 163, 164, 165). 

We give the enumeration of the places in the M shell in the form of a table  

The table shows not only those distributions of electrons and ground terms which have actually been ascertained from the spectra, but also those which have been determined by considerations of analogy the latter are indicated by an asterisk attached to the symbol of the element. The probability of correctness of the inferences from analogy is, however, very high. 

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