0.9 Bonding in metals and metal-non-metal salts  (Page 5/9)

What are the properties of NaCl? They are quite different than the properties of the metals we just discussed. First, NaCl is a solid crystal and it is not at all malleable. A crystal of NaCl, say “rock salt,” cannot be molded into whatever shape we choose. Rather, it is very brittle. Hit it with a hammer and, unlike a piece of metal, it shatters into tinier fragments of the crystal. Similarly, it is not ductile. It cannot be rolled or stretched into a wire or a thread. Second, solid NaCl is not an electrical conductor. Instead it insulates against the movement of current even when an electric potential is applied. We can immediately conclude from these observations that the bonding model we developed for a metal is not going to work to describe bonding in NaCl. We’ll have to start from scratch.

There are other interesting properties of NaCl. One is that it dissolves easily in water, which most metals do not. And when dissolved in water, the resulting solution conducts electricity. Somehow then a current can pass through the salt solution, meaning that there are charged particles dissolved in the solution which carry the movement of charge. These charged particles turn out to be ions, Na ⁺ and Cl ^- . Of course, this does not tell us whether there are ions in NaCl itself, since the interaction with the water molecules in the solution might change everything. Instead, we could try melting NaCl, so that we wind up with a liquid which is pure NaCl without any water. This takes a very high temperature, 808°C, indicating that there are strong forces at work in the solid NaCl crystal. When we melt NaCl, we find that the resulting liquid does in fact conduct electricity. Liquid NaCl thus consists of ions, Na ⁺ positive ions (“cations”) and Cl ^- negative ions (“anions”).

As a result, we should expect that these same ions exist in the solid NaCl. How can we reconcile the existence of ions in solid NaCl with fact that it does not conduct an electric current? The answer is that a current is charge in motion . Thus, the simple existence of an ion is not enough to carry a current. The ion must also be able to move, as electrons do in a metal, or as Na ⁺ and Cl ^- do when dissolved in water. The ions in the solid cannot move, at least not very far, as we have seen from the fact that NaCl is not malleable. In fact, the Na ⁺ and Cl ^- ions are basically fixed in place. From Coulomb’s law, we know that opposite charges are strongly attracted to each other. We can conclude then that the bonding in NaCl is due to the attraction of Na ⁺ cations to Cl ^- anions.

Why are there ions in the solid? The solid crystal itself is not electrically charged, so it isn’t clear why each Na atom has lost an electron and each Cl atom has gained an electron. Let’s look again at the properties of these very different kinds of atoms. We know that Na has low ionization energy, but it isn’t zero. It still does require a lot of energy to ionize the valence electron. We know that Cl has a much higher ionization energy. More importantly, we also know that Cl has a high electron affinity, which means that a lot of energy is released when an electron is added to a Cl atom.