If we now turn our attention to atoms with several electrons.degeneracy with respect to the orbital angular momentum quantum number l arises.The degeneracy with respect to the magnetic quantum number ml and the spin quantum number ms remain.Elements with the same principal quantum number n belong to the same shell (n=1,2,3,4…correspond to the K-,L-,M-,N-…shells) , where each shell accommodates a maximum of 2n2 elements.as is easily calculated on the basis of the possible combinations of the quantum numbers.Elements that are grouped energetically belong to one period.Periods encompass successively 2,8,8,18,18,32,32 elements as is immediately deduced from the scheme in Figure 2.9,and which is reflected by the Periodic Table (Appendix A4).This is an energetic order,which,essentially, is deduced empirically.A period is not defined by the principal quantum number as is sometimes assumed:elements with the same quantum number n are localized on “ diagonals ”of the term scheme and may belong to different periods.
Figure 2.9 Schematic representation of the electron distribution in the periods of the Periodic Table of the elements. Shells correspond to the principal quantum number and typically cover several periods. This clearly shows how the number of electrons, i.e. , elements, increases with increasing periods. The closed shells correspond to the noble gases indicated.