The smallest particle in matter with unique chemical properties is the atom. An atom is the basic building block of matter and consists of even smaller particles called protons, neutrons, and electrons. The specific combination and arrangement of these subatomic particles determine an atom's unique chemical properties, such as its reactivity, stability, and the ability to form bonds with other atoms. When different atoms combine, they form molecules, which are the structural units of most chemical compounds. Understanding the behavior and properties of atoms is fundamental to the study of chemistry.