What is the smallest whole unit of matter?

The smallest whole unit of matter is an atom. Atoms are the basic building blocks of matter and consist of subatomic particles such as protons, neutrons, and electrons. They can exist independently or can combine with other atoms to form molecules, which are collections of two or more atoms bonded together.

To clarify the other options: a molecule, while a valid unit of matter, is made up of two or more atoms bonded together, making it larger than an atom itself. A particle is a more general term that can refer to any small unit of matter, including atoms and subatomic particles, but does not define a specific smallest unit like an atom does. A compound is a type of molecule that consists of two or more different types of atoms chemically bonded, which again implies that it is larger than the simplest building block, the atom.

In summary, the atom is recognized as the fundamental unit of matter, making it the correct answer to the question posed.