What characteristic differentiates a crystal from an amorphous solid?

The defining characteristic that differentiates a crystal from an amorphous solid is the presence of a regular repeating pattern of atoms within a crystal structure. Crystalline solids are organized in a highly ordered arrangement, which extends in all three spatial dimensions. This regularity results in distinct geometric shapes and specific physical properties, such as well-defined melting points and predictable diffraction patterns when exposed to X-rays.

In contrast, amorphous solids lack this long-range order, resulting in a more random arrangement of atoms. This difference in atomic arrangement leads to variations in physical properties. For instance, while amorphous materials may exhibit different melting behaviors, they do not have a clear transition from solid to liquid but instead soften over a range of temperatures.

Understanding this distinction is fundamental in various fields, including materials science, geology, and chemistry, because it influences the properties and applications of different substances.