6.2. Electron Configuration and the Periodic Table http://www.ck12.org
TABLE6.2:(continued)
Period Number of Elements in Period Sublevels in Order of Filling
3 8 3 s 3 p
4 18 4 s 3 d 4 p
5 18 5 s 4 d 5 p
6 32 6 s 4 f 5 d 6 p
7 32 7 s 5 f 6 d 7 pRecall that the four different sublevels (s,p,d, andf) each consist of a different number of orbitals. Thessublevel
has one orbital, thepsublevel has three orbitals, thedsublevel has five orbitals, and thefsublevel has seven orbitals.
In the first period, only the 1ssublevel is being filled. Since all orbitals can hold two electrons, the entire first period
consists of just two elements. In the second period, the 2ssublevel, with two electrons, and the 2psublevel, with
six electrons, are being filled. Consequently, the second period contains eight elements. The third period is similar
to the second, except the 3sand 3psublevels are being filled. Because the 3dsublevel does not fill until after the 4s
sublevel, the fourth period contains 18 elements, due to the 10 additional electrons that can be accommodated by the
3 dorbitals. The fifth period is similar to the fourth. After the 6ssublevel fills, the 4fsublevel is populated with up
to 14 electrons. This is followed by the 5dand the 6psublevels. The total number of elements in the sixth period is
- The seventh period also contains 32 elements, most of which are too unstable to be found in nature. All 32 have
been detected or synthesized, although, for some of the later elements in this period, only a handful of atoms have
ever been made.
The period to which a given element belongs can easily be determined from its electron configuration. As an
example, consider the element nickel (Ni). Its electron configuration is [Ar]3d^84 s^2. The highest occupied principal
energy level is the fourth, as indicated by the 4 in the 4s^2 portion of the configuration. Therefore, nickel can be
found in the fourth period of the periodic table. TheFigure6.8 shows a version of the periodic table that includes
abbreviated electron configurations.
Based on electron configurations, the periodic table can be divided into blocks denoting which sublevel is in the
process of being filled. Thes,p,d, andfblocks are illustrated below (Figure6.9).
TheFigure6.9 also illustrates how thedsublevel is always one principal level behind the period in which that
sublevel occurs. In other words, the 3dsublevel fills during the fourth period. Thefsublevel is always two levels
behind. The 4fsublevel belongs to the sixth period.
Representative Elements
We will now examine each of these blocks in more detail. Thesandpsublevels for a given principal energy
level are filled during the correspondingly numbered period. For example, the 2sand 2psublevels fill during the
second period. Thes-block elements and the p-block elements are together called therepresentativeormain-group
elements.
The
Thes-block consists of the elements in Group 1 and Group 2, which are primarily composed of highly reactive
metals. Theelements in Group 1 (lithium, sodium, potassium, rubidium, cesium, and francium) are called thealkali
metals. All of the alkali metals have a singleselectron in their outermost principal energy. Recall that such electrons
are called valence electrons. The general form for the electron configuration of each alkali metal isns^1 , where the
nrefers to the highest occupied principal energy level. For example, the electron configuration of lithium (Li),