The electronic configuration of an atom is given by listing its subshells with the number of electrons in each subshell, as shown in Table
1 . Study the third column of complete electronic configurations carefully so you understand how electrons are added to the subshell of lowest energy until it reaches its capacity; then the subshell of the next energy level begins to be filled. The electrons on the highest numbered subshells are the
valence electrons, which comprise the valence shell of the atom.
TABLE 1
Electron Configurations and Oxidation Numbers
Element Name
Atomic Number
Electron Configuration
Valence Shell
Common Oxidation Numbers
Hydrogen
1
1
s1
1
s1
+1, −1
Helium
2
1
s2
1
s2
0
Lithium
3
1
s2 2
s1
2
s1
+1
Beryllium
4
1
s2 2
s2
2
s2
+2
Boron
5
1
s2 2
s2 2
p1
2
s2 2
p1
+3
Carbon
6
1
s2 2
s2 2
p2
2
s2 2
p2
+4, +2, −4
Nitrogen
7
1
s2 2
s2 2
p3
2
s2 2p3
+3, −3
Oxygen
8
1
s2 2
s2 2
p4
2
s2 2
p4
−2
Fluorine
9
1
s2 2
s2 2
p5
2
s2 2
p5s
−1
Neon
10
1
s2 2
s2 2
p6
2
s2 2
p6
0
Sodium
11
1
s2 2
s2 2
p6 3
s1
3
s1
+1
Magnesium
12
1
s2 2
s2 2
p6 3
s2
3
s2
+2
Aluminum
13
1
s2 2
s2 2
p6 3
s2 3
p1
3
s2 3
p1
+3
Silicon
14
1
s2 2
s2 2
p6 3
s2 3
p2
3
s2 3
p2
+4
Phosphorus
15
1
s2 2
s2 2
p6 3
s2 3
p3
3
s2 3
p3
+5, +3, −3
Sulfur
16
1
s2 2
s2 2
p6 3
s2 3
p4
3
s2 3
p4
+6, +4, +2, −2
Chlorine
17
1
s2 2
s2 2
p6 3
s2 3
p5
3
s2 3
p5
−1
Argon
18
1
s2 2
s2 2
p6 3
s2 3
p6
3
s2 3
p6
0
Potassium
19
1
s2 2
s2 2
p6 3
s2 3
p6 4
s1
4
s1
+1
Calcium
20
1
s2 2
s2 2
p6 3
s2 3
p6 4
s2
4
s2
+2
For brevity, many chemists record the electron configuration of an atom by giving only its outermost subshell, like 4
s1 for potassium or 4
s2 for calcium. These electrons are most distant from the positive nucleus and, therefore, are most easily transferred between atoms in chemical reactions. These are the valence electrons.
For ions, the valence equals the electrical charge. In molecules, the various atoms are assigned charge-like values so the sum of the oxidation numbers equals the charge on the molecule. For example, in the H2O molecule, each H has an oxidation number of +1, and the O is −2.
In Table
1 , the common oxidation numbers in the last column are interpreted as the result of either losing the valence electrons (leaving a positive ion) or gaining enough electrons to fill that valence subshell. Table
2 compares three ions and a neutral atom.
TABLE 2
Electron Configurations of Ions
Chemical Element
Valence Shell
Electron Transfer
Resulting Ion
Ion Configuration
Cl
3
s23
p5
gain 1
Cl−
1
s2 2
s2 2
p6 3
s2 3
p6
Ar
3
s23
p6
none
Ar0
1
s2 2
s2 2
p6 3
s2 3
p6
K
4
s1
lose 1
K+
1
s2 2
s2 2
p6 3
s2 3
p6
Ca
4
s2
lose 2
Ca2+
1
s2 2
s2 2
p6 3
s2 3
p6
The charges on the chlorine, potassium, and calcium ions result from a strong tendency of valence electrons to adopt the stable configuration of the inert gases, with completely filled electronic shells. Notice that the 3 ions have electronic configurations identical to that of inert argon.
Elements
Electron Configurations
