Electron Configurations and the Periodic Table

Our rather brief survey of electron configurations of the elements has taken us through the periodic table. We have seen that the electron configurations of elements are related to their location in the periodic table. The periodic table is structured so that elements with the same pattern of outer-shell (valence) electron configuration are arranged in columns. For example, the electron configurations for the elements in groups 2A and 3A are given in Table 6.4. We see that the 2A elements all have ns2 outer configurations while the 3A elements have ns2np1 configurations.

Earlier, in Table 6.2, we saw that the total number of orbitals in each shell is equal to n2: 1, 4, 9, or 16. Because each orbital can hold two electrons, each shell can accommodate up to 2n2 electrons: 2, 8, 18, or 32. We see that the beautiful structure of the periodic table reflects this orbital structure. The first row has two elements, the second and third rows have eight elements, the fourth and fifth rows have 18 elements, and the sixth row has 32 elements (including the lanthanide metals). Some of the numbers repeat because we reach the end of a row of the periodic table before a shell completely fills. For example, the third row has eight elements, which corresponds to filling the 3s and 3p orbitals. As noted earlier, the remaining orbitals of the third shell, the 3d orbitals, do not begin to fill until the fourth row of the periodic table (and after the 4s orbital is filled). Likewise, the 4d orbitals don't begin to fill until the fifth row of the table, and the 4f orbitals don't begin filling until the sixth row.

All of these observations are evident in the structure of the periodic table. For this reason we will emphasize that the periodic table is your best guide to the order in which orbitals are filled. You can easily write the electron configuration of an element based on its location in the periodic table. The pattern is summarized in Figure 6.29. Notice that the elements can be grouped in terms of the type of orbital into which the electrons are placed. On the left are two columns of elements. These elements, known as the alkali metals (group 1A) and alkaline earth metals (group 2A), are those in which the outer-shell s orbitals are being filled. On the right is a block of six columns. These are the elements in which the outermost p orbitals are being filled. The s block and the p block of the periodic table contain the representative (or main-group) elements. In the middle of the table is a block of ten columns that contains the transition metals. These are the elements in which the d orbitals are being filled. Below the main portion of the table are two rows that contain fourteen columns. These elements are often referred to as the f-block metals because they are the ones in which the f orbitals are being filled. Recall that the numbers 2, 6, 10, and 14 are precisely the number of electrons that can fill the s, p, d, and f subshells, respectively. Recall also that the 1s subshell is the first s subshell, the 2p is the first p subshell, the 3d is the first d subshell, and the 4f is the first f subshell.

FIGURE 6.29 Block diagram of the periodic table showing the groupings of the elements according to the type of orbital being filled with electrons.

Practice Exercise

What family of elements is characterized by having an ns2np2 outer-electron configuration? Answer: Group 4A

Sample Exercise 6.9

Write the electron configuration for the element bismuth, atomic number 83.

SOLUTION We can do this by simply moving across the periodic table one row at a time and writing the occupancies of the orbital corresponding to each row (refer to Figure 6.29).

First row 1s2

Second row 2s22p6

Third row 3s23p6

Fourth row 4s23d104p6

Fifth row 5s24d105p6

Sixth row 6s24f145d106p3

Total: 1s22s22p63s23p63d104s24p64d104f145s25p65d106s26p3

Note that 3 is the lowest possible value that n may have for a d orbital, and that 4 is the lowest possible value of n for an f orbital.

The total of the superscripted numbers should equal the atomic number of bismuth, 83. The electrons may be listed, as shown here, in the order of increasing major quantum number. However, it is equally correct to list the orbitals in an electron configuration in the order in which they are read from the periodic table: 1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p3.

It is a simple matter to write the abbreviated electron configuration of an element using the periodic table. First locate the element of interest (in this case element 83) and then move backward until the first noble gas is encountered (in this case Xe, element 54). Thus the inner core is [Xe]. The outer electrons are then read from the periodic table as before. Moving from Xe to Cs, element 55, we find ourselves in the sixth row. Moving across this row to Bi gives us the outer electrons. Thus the abbreviated electron configuration is as follows: [Xe]6s24f145d106p3 or [Xe]4f145d106s26p3.

Practice Exercise

Use the periodic table to write the electron configurations for the following atoms by giving the appropriate noble-gas inner core plus the electrons beyond it: (a) Co (atomic number 27); (b) Te (atomic number 52). Answers: (a) [Ar]4s23d7 or [Ar]3d74s2; (b) [Kr]5s24d105p4 or [Kr]4d105s25p4