Let us return to the properties of ionic compounds and see how this molecular-level (microscopic) model of bonding explains their properties. First, their high melting points arise from the fact that enough energy must be supplied so that multiple (strong) coulombic interactions (recall each cation is surrounded by six anions and vice versa) between the ions must be overcome. In contrast for water, it is only the intermolecular forces between molecules that must be overcome to melt ice; IMFs are significantly weaker than full ionic interactions. Similarly it takes even more energy to vaporize, that is to change NaCl liquid into a gas.
Now let us predict the melting points of different ionic compounds. Remember that the force between the ions is a Coulombic attraction: F α (q+ x q– ) / r2, where q+ and q– are the charges on the ions, and r is the distance between them. This equation tells us that as the charge on the ions increases, so does the force of attraction, but as the distance between them increases, the force of attraction decreases. That is, the coulombic attraction should be larger for small, highly charged ions, and this should be reflected in the melting points of ionic compounds. Even when we don't factor in the size of the ions, q1 x q2 = 4 which means that the attractive forces for CaO should be on the order of 4 times those for NaCl. Indeed, the melting point of calcium oxide (CaO) which has q1 = 2+ and q2 = 2– is 2,572°C.
Questions to Answer
1. Draw a molecular level picture of liquid water, and a molecular level picture of liquid sodium chloride. Use this picture to explain why it takes more energy to melt solid salt than it does to melt solid water.
2. Arrange these ionic compounds in order of increasing melting point: NaCl, KBr, CaO, Al2O3. Look up your answers and see if your predictions were correct.
3. Arrange these materials in order of increasing melting point: CH4, MgBr2, HF, C(diamond). Look up your answers and see if your predictions were correct.
4. What do you think happens to the size of the particle when a chlorine atom gains an electron to become a chloride ion? (hint recall that the size of an atom depends on the balance between the attractions between the electrons and the nucleus, and the repulsions between the electrons)
5. What do you think happens to the size of the particle when a sodium atom loses an electron to become a sodium ion?