What is an atom?

An atom is indeed the basic structural unit of all matter, serving as the foundational building block from which everything is composed. Every solid, liquid, gas, and plasma is made up of atoms, which themselves consist of a nucleus containing protons and neutrons, surrounded by electrons. This concept is essential to understanding the nature of materials and their properties, including gems and minerals.

The other options describe different concepts: minerals, for instance, are specific types of naturally occurring inorganic substances, and while gemstones are crystallized minerals, they are complex structures made up of numerous atoms bonded together. A molecule refers to a group of atoms bonded together, which is distinct from the single, indivisible unit of an atom. Finally, the arrangement of crystals refers to a specific geometric alignment of atoms or molecules within a solid, which does not define what an atom is. Thus, recognizing that an atom is the fundamental building block of all matter underscores its significance in the study of chemistry and materials science.