In a previous posting, I wrote that the microscopic world, in this case implying inter-atomic distances, generates an approximation of the macroscopic, mechanical properties of matter.
What any alert reader should notice, is that in order for this theory to be true, it actually needs to lead to an exact result at some point, and not just to approximate results. And so the question which should follow is, ‘Why only an approximation, the way it was described?’
There is a family of answers to that question, which starts with the fact that not all solids are covalent solids. I was taught that there exist essentially three types of solids:
- Molecular Solids,
- Covalent Solids,
- Ionic Solids.
I feel that the WiKiPedia article I linked to in this list, gives a good explanation for what Molecular Solids are, and also gives links to the other types of solids. If the reader has serious questions, I recommend he read that WiKi next; they explain certain details better than I can.
At the same time, solids which I was taught were covalent solids, are really just a combination of molecular and covalent solids, due to the way molecules could be linked in certain directions, but not linked in other directions, in 3D. This is why the WiKi describes those types of solids as ‘mesh-solids’.
Organic polymers are extreme examples of meshes, while certain structural materials such as beryllium are completely different, being highly covalent, and being much stronger therefore, than organic polymers.
Another reason for which my first description is only an approximation, is the existence of thermal agitation. This means that individual nuclei are always in motion, even if the macroscopic body is not noticeably in motion. Furthermore, due to the involvement of Quantum Mechanics, heat can take the form of transitions between discrete states, instead of all the heat being stored, just as the continuous agitation of the nuclei. Hence, molecules which have a greater number of QM states to occupy, at any given temperature, will also store more heat, as their temperature changes, and will therefore also have greater specific heat. If heat was just the kinetic energy of the nuclei, we should find that all matter have very predictable properties of specific heat, just a function of atomic density, when in fact this is not so.
And, the velocities associated with thermal agitation at room temperature, are often underestimated. They can be enough to break the bonds between molecules by themselves, which is also a reason ‘why ice melts at room temperature’.