Page 81 - Theory and Problems of BEGINNING CHEMISTRY

70 CHEMICAL BONDING [CHAP. 5

−
− +
−
− + + + −
+ − +
− + Cl − −
Na + − +
+
− + −
− +
+
− + −

Fig. 5-1. Ball-and-stick model of the sodium chloride structure
The lines are not covalent bonds (Sec. 5.6), only indications of the positions of the ions. Sodium chloride is the
most common compound with this structure, which is therefore called the sodium chloride structure. Potassium
chloride also has this structure.

EXAMPLE 5.5. Predict the charge on a magnesium ion and that on a fluoride ion, and deduce the formula of magnesium
fluoride.
Ans. The electron configuration of a magnesium atom is
6
2
2
2
Mg 2 8 2 (1s 2s 2p 3s )
By losing two electrons, a magnesium atom attains the electronic configuration of a neon atom and thereby acquires
a charge of 2+.
2
2
6
Mg 2+ 2 8 (1s 2s 2p )
A fluorine atom has the configuration
2
2
5
F 2 7 (1s 2s 2p )
By gaining one electron, the fluorine atom attains the electronic configuration of neon and also attains a charge of
1−. The two ions expected are therefore Mg 2+ and F . Since magnesium fluoride as a whole cannot have any net
−
charge, there must be two fluoride ions for each magnesium ion; hence, the formula is MgF 2 .
The ionic nature of these compounds can be shown by experiments in which the charged ions are made to
carry an electric current. If a compound that consists of ions is dissolved in water and the solution is placed
between electrodes in an apparatus like that shown in Fig. 5-2, the solution will conduct electricity when the
electrodes are connected to the terminals of a battery. Each type of ion moves toward the electrode having a charge
opposite to that of the ion. Positively charged ions are called cations (pronounced “cat’-ions”), and negatively
charged ions are called anions (pronounced “an’-ions”). Thus, cations migrate to the negative electrode, and
anions migrate to the positive electrode. In order to have conduction of electricity, the ions must be free to move.

To battery
Wire

Positive electrode Negative electrode
Cl −

Ca 2+
Cl −

Fig. 5-2. Conducting by ions in solution