How is an atom’s electron configuration related to its chemical behavior, and why do elements with similar behavior occur in the same group of the periodic table?
As shown in Figure 2.7, the periodic table can be divided into four regions, or blocks, of elements according to the electron shells and subshells occupied by the subshell filled last.
• The main group 1A and 2A elements on the left side of the table (plus He) are called the s-block elements because an s subshell is filled last in these elements.
• The main group 3A–8A elements on the right side of the table (except He) are the p-block elements because a p subshell is filled last in these elements.
• The transition metals in the middle of the table are the d-block elements because a d subshell is filled last in these elements.
• The inner transition metals detached at the bottom of the table are the f-block elements because an f subshell is filled last in these elements.
Thinking of the periodic table as outlined in Figure 2.7 provides a simple way to remember the order of orbital filling shown previously in Figure 2.6. Beginning at the top left corner of the periodic table, the first row contains only two elements (H and He) because only two electrons are required to fill the s orbital in the first shell, 1s2. The second row begins with two s-block elements (Li and Be) and con- tinues with six p-block elements (B through Ne), so electrons fill the next available s orbital (2s) and then the first available p orbitals (2p). The third row is similar to the second row, so the 3s and 3p orbitals are filled next. The fourth row again starts with 2 s-block elements (K and Ca) but is then followed by 10 d-block elements (Sc through Zn) and 6 p-block elements (Ga through Kr). Thus, the order of orbital filling is 4s followed by the first available d orbitals (3d) followed by 4p. Continuing through successive rows of the periodic table gives the entire filling order, identical to that shown in Figure 2.6.
1sS2sS2pS3sS3pS4sS3dS4pS5sS 4dS5pS6sS4fS5dS6pS7sS5fS6dS 7p
But why do the elements in a given group of the periodic table have similar prop- erties? The answer emerges when you look at Table 2.4, which gives electron config- urations for elements in the main groups 1A, 2A, 7A, and 8A. Focusing only on the s-Block element A main group
element that results from the filling of an s orbital.
p-Block element A main group element that results from the filling of p orbitals.
d-Block element A transition metal element that results from the filling of d orbitals.
f-Block element An inner transition metal element that results from the filling of f orbitals.
1s 1s
2s 3s 4s 5s 6s 7s
3d 4d 5d 6d
2p 3p 4p 5p 6p 7p 2
Begin here
6 10
4f 5f
s block
14
p block d block f block
▶ Figure 2.7
The blocks of elements in the periodic table correspond to filling the different types of subshells.
Beginning at the top left and going across successive rows of the periodic table provides a method for remem- bering the order of orbital filling:
1sS2sS2pS3sS3pS4sS 3dS4p, and so on.
S E C T I O N 2 . 8 Electron Configurations and the Periodic Table 63
electrons in the outermost shell, or valence shell, elements in the same group of the periodic table have similar electron configurations in their valence shells. The group 1A elements, for example, all have one valence electron, ns1 (where n represents the num- ber of the valence shell: n = 2 for Li; n = 3 for Na; n = 4 for K; and so on). The group 2A elements have two valence electrons 1ns22; the group 7A elements have seven valence electrons 1ns2 np52; and the group 8A elements (except He) have eight valence electrons 1ns2 np62. You might also notice that the group numbers from 1A through 8A give the numbers of valence electrons for the elements in each main group. It is worth noting that the valence electrons are those in the outermost shell—not necessarily in the orbitals that were filled last!
What is true for the main group elements is also true for the other groups in the periodic table: atoms within a given group have the same number of valence electrons and have similar electron configurations. Because the valence electrons are the most loosely held, they are the most important in determining an element’s properties. Similar electron configurations thus explain why the elements in a given group of the periodic table have similar chemical behavior.
Valence shell The outermost electron shell of an atom.
Valence electron An electron in the valence shell of an atom.
Table 2.4 Valence-Shell Electron Configurations for Group 1A, 2A, 7A, and 8A Elements
Group Element
Atomic Number
Valence-Shell Electron Configuration
1A Li (lithium) 3 2s1
Na (sodium) 11 3s1
K (potassium) 19 4s1
Rb (rubidium) 37 5s1
Cs (cesium) 55 6s1
2A Be (beryllium) 4 2s2
Mg (magnesium) 12 3s2
Ca (calcium) 20 4s2
Sr (strontium) 38 5s2
Ba (barium) 56 6s2
7A F (fluorine) 9 2s2 2p5
Cl (chlorine) 17 3s2 3p5
Br (bromine) 35 4s2 4p5
I (iodine) 53 5s2 5p5
8A He (helium) 2 1s2
Ne (neon) 10 2s2 2p6
Ar (argon) 18 3s2 3p6
Kr (krypton) 36 4s2 4p6
Xe (xenon) 54 5s2 5p6
LOOKING AHEAD We have seen that elements in a given group have similar chemical behavior because they have similar valence electron configurations, and that many chemical properties exhibit periodic trends across the periodic table. The chemical behavior of nearly all the elements can be predicted based on their position in the periodic table, and this will be examined in more detail in Chapters 3 and 4. Similarly, the nuclear behavior of the different isotopes of a given element is related to the configuration of the nucleus (that is, the number of neutrons and protons) and will be examined in Chapter 11.
Worked Example 2.9 Electron Configurations: Valence Electrons
Write the electron configuration for the following elements, using both the complete and the shorthand notations. Indicate which electrons are the valence electrons.
(a) Na (b) Cl (c) Zr
ANALYSIS Locate the row and the block in which each of the elements is found in Figure 2.7. The location can be used to determine the complete electron configura- tion and to identify the valence electrons.
SOLUTION
(a) Na (sodium) is located in the third row and in the first column of the s-block.
Therefore, all orbitals up to the 3s are completely filled, and there is one electron in the 3s orbital.
Na: 1s2 2s2 2p6 3s1 or 3Ne4 3s1 (valence electrons are underlined2 (b) Cl (chlorine) is located in the third row and in the fifth column of the
p-block.
Cl: 1s2 2s2 2p6 3s2 3p5 or 3Ne4 3s2 3p5
(c) Zr (zirconium) is located in the fifth row and in the second column of the d-block. All orbitals up to the 4d are completely filled, and there are 2 electrons in the 4d orbitals. Note that the 4d orbitals are filled after the 5s orbitals in both Figures 2.6 and 2.7.
Zr: 1s2 2s2 2p6 3s1 3p6 4s2 3d10 4p6 5s2 4d2 or 3Kr4 5s2 4d2
Worked Example 2.10 Electron Configurations: Valence-Shell Configurations
Using n to represent the number of the valence shell, write a general valence-shell configuration for the elements in group 6A.
ANALYSIS The elements in group 6A have 6 valence electrons. In each element, the first two of these electrons are in the valence s subshell, giving ns2, and the next four electrons are in the valence p subshell, giving np4.
SOLUTION
For group 6A, the general valence-shell configuration is ns2 np4.
Worked Example 2.11 Electron Configurations: Inner Shells versus Valence Shell How many electrons are in a tin atom? Give the number of electrons in each shell.
How many valence electrons are there in a tin atom? Write the valence-shell con- figuration for tin.
ANALYSIS The total number of electrons will be the same as the atomic number for tin 1Z = 502. The number of valence electrons will equal the number of electrons in the valence shell.
SOLUTION
Checking the periodic table shows that tin has atomic number 50 and is in group 4A. The number of electrons in each shell is
Shell number: 1 2 3 4 5
Number of electrons: 2 8 18 18 4
As expected from the group number, tin has 4 valence electrons. They are in the 5s and 5p subshells and have the configuration 5s2 5p2.
S E C T I O N 2 . 9 Electron-Dot Symbols 65
PROBLEM 2.22
Write the electron configuration for the following elements, using both the complete and the shorthand notations. Indicate which electrons are the valence electrons.
(a) F (b) Al (c) As
PROBLEM 2.23
Identify the group in which all the elements have the valence-shell configuration ns2. PROBLEM 2.24
For chlorine, identify the group number, give the number of electrons in each occu- pied shell, and write its valence-shell configuration.
KEY CONCEPT PROBLEM 2.25
Identify the group number, and write the general valence-shell configuration (for example, ns1 for group 1A elements) for the elements indicated in red in the following periodic table.
Electron-dot symbol An atomic symbol with dots placed around it to indicate the number of valence electrons.