What do you call a solid in which the atoms are arranged in a repeating pattern?

A solid in which the atoms are arranged in a repeating pattern is known as a crystal. Crystals possess a well-defined geometric shape and structure due to the orderly arrangement of their constituent atoms or molecules. This repeating pattern leads to distinct physical properties such as symmetry, hardness, and clarity, which are often characteristic of crystalline materials.

For instance, common examples of crystals include table salt (sodium chloride) and quartz, both of which showcase this ordered arrangement. This structural organization contrasts with other types of solids, such as amorphous solids, which do not have a long-range order and lack a defined shape, or mixtures, which are combinations of two or more substances that retain their individual properties. Polymers, while they may have repeated structural units, do not necessarily form a regular three-dimensional structure like crystals do. Thus, the defining characteristic of a crystal is its repeating atomic pattern, which gives rise to its unique and orderly form.