Final answer:
Amorphous solids have a random, disordered atomic arrangement with no sharp melting point, while crystalline solids are organized in a repeating three-dimensional lattice, with distinct melting points and directional properties.
Step-by-step explanation:
The fundamental difference between an amorphous solid and a crystalline solid lies in the arrangement of their atoms on the microscopic level. In amorphous solids, atoms or molecules are arranged randomly and lack a long-range order. This results in properties such as not having a definite melting point and being isotropic, meaning their properties are the same in all directions.
Conversely, a crystalline solid has a well-ordered atomic structure that repeats in a three-dimensional pattern known as a lattice. This regularity gives rise to clear-cut planes of cleavage, sharp melting points, and anisotropy, where the properties may vary in different directions. Crystalline solids include various types such as ionic, molecular, covalent network, and metallic, each with unique characteristics due to the nature of their bonding and structure.