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] ns^1 , 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] ns^2 ; 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] ns^2 (n − 1)d^1 . (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