ELECTRONIC STRUCTURE AND THE PERIODIC TABLE

4.8. ELECTRONIC STRUCTURE AND THE PERIODIC TABLE

The arrangement of electrons in successive energy levels in the atom provides an explanation of the periodicity of the elements, as found in the periodic table. The charges on the nuclei of the atoms increase in a regular manner as the atomic number increases. Therefore, the number of electrons surrounding the nucleus increases also. The number and arrangement of the electrons in the outermost shell of an atom vary in a periodic manner (compare Table 4-6). For example, all the elements in Group IA—H, Li, Na, K, Rb, Cs, Fr—corresponding to the elements that begin a new row or period, have electronic configurations with a single electron in the outermost shell, specifically an s subshell.

The noble gases, located at the end of each period, have electronic configurations of the type ns 2 np 6 , where n represents the number of the outermost shell. Also, n is the number of the period in the periodic table in which the element is found.

Since atoms of all elements in a given group of the periodic table have analogous arrangements of electrons in their outermost shells and different arrangements from elements of other groups, it is reasonable to conclude that the outermost electronic configuration of the atom is responsible for the chemical characteristics of the element. Elements with similar arrangements of electrons in their outer shells will have similar properties. For example, the formulas of their oxides will be of the same type. The electrons in the outermost shells of the atoms are referred to as valence electrons. The outermost shell is called the valence shell.

As the atomic numbers of the elements increase, the arrangements of electrons in successive energy levels vary in a periodic manner. As shown in Fig. 4-5, the energy of the 4s subshell is lower than that of the 3d subshell. Therefore, at atomic number 19, corresponding to the element potassium, the 19th electron is found in the 4s subshell rather than the 3d subshell. The fourth shell is started before the third shell is completely filled. At atomic number 20, calcium, a second electron completes the 4s subshell. Beginning with atomic number 21 and continuing through the next nine elements, successive electrons enter the 3d subshell. When the 3d subshell is complete, the following electrons occupy the 4 p subshell through atomic number 36, krypton. In other words, for elements 21 through 30, the last electrons added are found in the 3d subshell rather than in the valence shell. The elements Sc through Zn are called transition elements, or d block elements. A second series of transition elements begins with yttrium, atomic number 39, and includes 10 elements. This series corresponds to the placement of

10 electrons in the 4d subshell. The elements may be divided into types (Fig. 4-8), according to the position of the last electron added to those present in the preceding element. In the first type, the last electron added enters the valence shell. These elements are called the main group elements. In the second type, the last electron enters a d subshell in the next-to-last shell. These elements are the transition elements. The third type of elements has the last electron

CHAP. 4]

ELECTRONIC CONFIGURATION OF THE ATOM

enter the f subshell in the n − 2 shell—the second shell below the valence shell. These elements are the inner transition elements .

Main groups

Transition groups

Main groups

Inner transition groups Fig. 4-8. Periodic table as an aid to assigning electronic configurations

An effective way to determine the detailed electronic configuration of any element is to use the periodic table to determine which subshell to fill next. Each s subshell holds a maximum of 2 electrons; each p subshell holds a maximum of 6 electrons; each d subshell holds a maximum of 10 electrons; and each f subshell holds

a maximum of 14 electrons (Table 4-5). These numbers match the numbers of elements in a given period in the various blocks. To get the electronic configuration, start at hydrogen (atomic number = 1) and continue in order of atomic number, using the periodic table of Fig. 4-8.

EXAMPLE 4.12. Using the periodic table, determine the detailed electronic configuration of magnesium. Ans.

Starting at hydrogen, we put two electrons into the 1s subshell, then two more electrons into the 2s subshell. We continue (at atomic number 5) with the 2 p subshell, and enter six electrons there, corresponding to the six elements (elements 5 to 10, inclusive) in that p block of the periodic table. We have two more electrons to put into the 3s subshell, which is next. Thus, we always start at hydrogen, and we end at the element required. The number of electrons that we add to each subshell is equal to the number of elements in the block of the periodic table. In this case, we added electrons from hydrogen to magnesium, following the atomic numbers in order, and we got a

configuration 1s 2 2s 2 2p 6 3s 2 .

EXAMPLE 4.13. Write the detailed electronic configurations for K, S, and Y. Ans.

In each case, the superscripts total to the atomic number of the element. EXAMPLE 4.14. Determine the detailed electronic configuration of Gd, atomic number 64.

Ans.

Gd 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 5d 1 4f 7

We note that the 5d subshell started before the 4 f subshell, but only one electron entered that shell before the 4 f subshell started. Indeed, the periodic table predicts this correct configuration for Gd better than the n + l rule or other common memory aids.

Instead of writing out the entire electronic configuration of an atom, especially an atom with many electrons, we sometimes abbreviate the configuration by using the configuration of the previous noble gas and represent the rest of the electrons explicitly. For example, the full configuration of cobalt can be given as

Co

1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 7

62 ELECTRONIC CONFIGURATION OF THE ATOM

[ CHAP. 4

Alternatively, we can use the configuration of the previous noble gas, Ar, and add the extra electrons:

Co

[Ar] 4s 2 3d 7

To determine the electronic configuration in this manner, start with the noble gas of the previous period and use the subshell notation from only the period of the required element. Thus, for Co, the notation for Ar (the previous

noble gas) is included in the square brackets, and the 4s 2 3d 7 is obtained across the fourth period. It is suggested that you do not use this notation until you have mastered the full notation. Also, on examinations, use the full notation unless the question or the instructor indicates that the shortened notation is acceptable.