Page 71 - Theory and Problems of BEGINNING CHEMISTRY

60 ELECTRONIC CONFIGURATION OF THE ATOM [CHAP. 4

5s 4d
4s 3d
3s 3p
2p
2s
1s
Fig. 4-7. Vessel with layered base

4.8. ELECTRONIC STRUCTURE AND THE PERIODIC TABLE
Thearrangementofelectronsinsuccessiveenergylevelsintheatomprovidesanexplanationoftheperiodicity
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.

H 1s 1
2
Li 1s 2s 1
2
2
6
Na 1s 2s 2p 3s 1
6
2
6
2
2
K 1s 2s 2p 3s 3p 4s 1
2
6
10
6
2
2
6
2
Rb 1s 2s 2p 3s 3p 4s 3d 4p 5s 1
2
2
2
6
6
10
10
2
6
2
6
Cs 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 1
10
6
2
6
2
2
14
10
6
10
6
2
2
6
2
Fr 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 1
2
6
The noble gases, located at the end of each period, have electronic configurations of the type ns np , 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 4p 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