Crystalline vs. Amorphous Substances (Crystal Structure) When studying solids in physics and chemistry, one of the key ways we classify them is by examining how their atoms or molecules are arranged. This arrangement is referred to as the crystal structure, and it plays a big role in determining the physical properties of the substance. Solids can be broadly classified into two categories based on their internal structure: crystalline and amorphous substances. Crystalline Substances Crystalline