|There's a lot of formalism in chemistry. We identify particular groups of compounds with characteristic spectra, characteristic solubility, and characteristic crystal structure. When they are isolated and purified into a crystal, you can actually refer to a diagram of the architecture of the molecule and how they're packed together to form that crystal. Because of this architecture, much like the architecture of a building, you have a mental image of the molecule. The geometry of the compound is how it's stuck together.