Valence Electrons and the Periodic Table
The valence electrons of an atom are those electrons that are in its outer energy shell or that are available for bonding. The visual layout of the periodic table is convenient for determining the electron configuration of an atom (especially the valence electron configuration); this provides a quick alternative to the methods described in the previous chapter.
Elements in the leftmost column (Group 1 or IA) all have a single s electron in their valence shell; their electron configuration can therefore be represented as [X] ns1, where [X] designates the electron configuration of the noble gas element (see p. 71) that immediately precedes it and is abbreviated this way because these electrons are core electrons that do not generally participate in chemical reactions and are hence uninteresting from a chemical perspective. Elements in the second column (Group 2 or IIA) have electronic configurations [X] ns2; their valence electrons are the two electrons in the outermost s subshell.
The next block of elements (elements in the next 10 columns, not including the 4f lanthanide and 5f actinide series) are all known as transition elements and have electrons in the d subshell; just how many they have depends on exactly which column they are in. Elements in the third column (Group 3 or IIIA), for example, have configurations [X] ns2 (n − 1)d1. (Note that the principal quantum number for the d subshell is one less than that for the s subshell—remember, for example, how after filling the 3p subshell, one fills the lower-energy 4s orbital first before “going back” to fill the 3d subshell.) Their valence electrons are those in the outermost s subshell and in the d subshell of the next-to-outermost energy shell. For the inner transition elements, the valence electrons are those in the s subshell of the outermost energy shell, the d subshell of the next-to-outermost energy shell, and the f subshell of the energy shell two levels below the outermost shell.
The last six columns of the periodic table contain elements with s and p valence electrons.