For our practical purposes, metallurgists working at the level of my daily activities don’t need to be concerned with anything more complicated than I have just explained. The details of the shape and electrical charge of the electron clouds have a strong effect on the thermodynamic driving force that we discussed in Part 1. The electron clouds' behavior also has a strong effect on the corrosion resistance of a material. Finally, the electron clouds have a very strong effect on determining which category (metal, polymer, ceramic) will best describe the solid.
The important thing to understand is that the outer electrons in metallic atoms, when present mainly with other metallic atoms, are held very loosely by the nuclei of the atoms. This creates the possibility for electrical conductivity and also malleability, two of the key properties of metals. In ceramic materials, the electrons are shared by multiple atoms as well, but the electrons are not as free to move around within the larger solid component. As a group, ceramic materials are much less electrically conductive than metals, and they are brittle, not malleable. If you force the electrons apart from the one they are in touch with, it’s over. They don’t readily choose another, as happens in metals.
In polymeric materials, we have a complex hierarchy of atomic bonds that result from the charge and shape of the electron cloud. We will have atom-to-atom bonds resulting from electrons that are shared by a limited number of other atoms. These electrons are not free, and they link large numbers of atoms into a chain or network. In polymers, we have another layer of structure related to neighboring atoms that are not directly in contact with each other within the polymer chain or network. These neighbor-to-neighbor links are generally weaker than the in-chain or in-network links but still affect the overall behavior of the material.
I really wasn’t planning to take so much time talking about atomic structure, because failure analysis generally relies more directly on microstructure, two levels up. But when we have worked in a field for a long time, I think most of us forget the details of how we learned what we know. Yet, understanding those details and how they interact with each other is what allows us to develop our expertise and help others to gain a little more useful understanding of our subjects.
To close this part, I thought I would show the shape of the electron probability cloud for iron. Unfortunately, I couldn’t find one. Our taxpayer dollars provided this image, which is an artist’s conception of the shape of the electron probability cloud for neon, an atom that generally only finds itself as part of a diatomic noble-gas molecule. We can see that although Democritus didn’t understand the full picture of what we think we know today, he wasn’t completely off base with his ideas of the shape of the atom being a key to the behavior of the material.
I plan to take a look at the next levels of structure in Part 4.