What is the smallest unit of matter that can exist independently?

The smallest unit of matter that can exist independently is an atom. Atoms are the basic building blocks of all matter and are composed of subatomic particles: protons, neutrons, and electrons. Each element on the periodic table is made up of atoms, and they can exist as single entities with their distinct properties.

Atoms have a unique number of protons that define them as a specific element, allowing them to participate in various chemical reactions. For example, a hydrogen atom can exist independently, while a molecule (which may consist of two or more atoms bonded together) or a compound (which is a type of molecule made up of different elements) cannot exist without combining their respective atoms. Similarly, "particle" is a broader term that can refer to any small unit of matter, including atoms but also including larger components like molecules or aggregates of atoms. Thus, the specificity of "atom" makes it the correct answer in this context.