I - Crystalline solids

Summary

If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis that all things are made of atoms – little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, there is an enormous amount of information about the world, if just a little imagination and thinking are applied.

(R. P. Feynman, The Feynman Lectures on Physics)

Solids are the physical objects with which we come into contact continuously in our everyday life. Solids are composed of atoms. This was first postulated by the ancient Greek philosopher Demokritos, but was established scientifically in the 20th century. The atoms (ατομα = indivisible units) that Demokritos conceived bear no resemblance to what we know today to be the basic units from which all solids and molecules are built. Nevertheless, this postulate is one of the greatest feats of the human intellect, especially since it was not motivated by direct experimental evidence but was the result of pure logical deduction.

There is an amazing degree of regularity in the structure of solids. Many solids are crystalline in nature, that is, the atoms are arranged in a regular three-dimensional periodic pattern. There is a wide variety of crystal structures formed by different elements and by different combinations of elements.