The format of the periodic table
المؤلف:
Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong
المصدر:
Shriver and Atkins Inorganic Chemistry ,5th E
الجزء والصفحة:
ص21-22
2025-08-18
437
The format of the periodic table
Key points: The blocks of the periodic table reflect the identity of the orbitals that are occupied last in the building-up process. The period number is the principal quantum number of the valence shell. The group number is related to the number of valence electrons. The layout of the periodic table reflects the electronic structure of the atoms of the elements (Fig. 1.22). We can now see, for instance, that a block of the table indicates the type of subshell currently being occupied according to the building-up principle. Each period, or row, of the table corresponds to the completion of the s and p subshells of a given shell. The period number is the value of the principal quantum number n of the shell which ac-cording to the building-up principle is currently being occupied in the main groups of the table. For example, Period 2 corresponds to the n 2 shell and the filling of the 2s and 2p subshells.
The group numbers, G, are closely related to the number of electrons in the valence shell, the outermost shell of the atom. In the ‘1–18’ numbering system recommended by IUPAC:
Figure 1.22 The general structure of the periodic table. Compare this template with the complete table inside the front cover for the identities of the elements that belong to each block.
For the purpose of this expression, the ‘valence shell’ of a d-block element consists of the ns and (n-1)d orbitals, so a Sc atom has three valence electrons (two 4s and one 3d electron). The number of valence electrons for the p-block element Se (Group 16) is 16 which corresponds to the configuration s2p4.
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