Page 80 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 5] CHEMICAL BONDING 69

The charge on the nucleus and the number of electrons in the valence shell determine the chemical properties
of the atom. The electronic configurations of the noble gases (except for that of helium) correspond to a valence
shell containing eight electrons—a very stable configuration called an octet. Atoms of other main group elements
tend to react with other atoms in various ways to achieve the octet, as discussed in the next sections. The tendency
to achieve an octet of electrons in the outermost shell is called the octet rule. If the outermost shell is the first
shell, that is, if there is only one shell occupied, then the maximum number of electrons is two. A configuration
of two electrons in the first shell, with no other shells occupied by electrons, is stable.

5.4. IONS
The electronic configuration of a potassium atom is
2
6
1
6
2
2
K 2 8 8 1 (1s 2s 2p 3s 3p 4s )
It is readily seen that if a potassium atom were to lose one electron, the resulting species would have the
configuration
6
2
6
2
2
0
K + 2 8 8 0 (1s 2s 2p 3s 3p 4s )
or more simply
2
2
6
6
2
K + 2 8 8 (1s 2s 2p 3s 3p )
The nucleus of a potassium atom contains 19 protons, and if there are only 18 electrons surrounding the nucleus
after the atom has lost one electron, the atom will have a net charge of 1+. An atom (or group of atoms) that
contains a net charge is called an ion. In chemical notation, an ion is represented by the symbol of the atom
+
with the charge indicated as a superscript to the right. Thus, the potassium ion is written K . The potassium
ion has the same configuration of electrons that an argon atom has (Example 5.3b). Ions that have the electronic
configurations of noble gases are rather stable. Note the very important differences between a potassium ion and
+
+
an argon atom—the different nuclear charges and the net 1+ charge on K . The K ion is not as stable as the
Ar atom.
EXAMPLE 5.4. What is the electronic configuration of a chloride ion, obtained by adding an electron to a chlorine atom?
Ans. The electronic configuration of a chlorine atom is
2
5
6
2
2
Cl 2 8 7 (1s 2s 2p 3s 3p )
Upon gaining an electron, the chlorine atom achieves the electronic configuration of argon:
2
2
6
6
2
Cl − 2 8 8 (1s 2s 2p 3s 3p )
However, since the chlorine atom contains 17 protons in its nucleus and now contains 18 electrons outside the
nucleus, it has a net negative charge; it is an ion. The ion is designated Cl and named the chloride ion.
−
Compounds—even ionic compounds—have no net charge. In the compound potassium chloride, there are
potassium ions and chloride ions; the oppositely charged ions attract one another and form a regular geometric
arrangement, as shown in Fig. 5-1. This attraction is called an ionic bond. There are equal numbers of K ions
+
and Cl ions, and the compound is electrically neutral. It would be inaccurate to speak of a molecule of solid
−
potassium chloride or of a bond between a specific potassium ion and a specific chloride ion. The substance KCl
is extremely stable because of (1) the stable electronic configurations of the ions and (2) the attractions between
the oppositely charged ions.
The electronic configurations of ions of many main group elements and even a few transition elements can
be predicted by assuming that the gain or loss of electrons by an atom results in a configuration analogous to that
of a noble gas—a noble gas configuration, which contains an octet of electrons in the outermost shell. Not all
the ions that could be predicted with this rule actually form. For example, few monatomic ions have charges of
4+, and no monatomic ions have charges of 4−. Moreover, nonmetal atoms bond to each other in another way
(Sec. 5.6).