What is the simplest particle of matter?

The simplest particle of matter is an atom. Atoms are the basic building blocks of matter, consisting of protons, neutrons, and electrons. They cannot be broken down into simpler substances by chemical means while still retaining their identity as a specific element.

This concept underlies the foundation of chemistry and physics, where different elements are represented by different types of atoms. Each element on the periodic table corresponds to a different type of atom, defined by its number of protons.

In contrast, compounds are substances formed when two or more different types of atoms bond together in fixed proportions. Molecules, which can consist of more than one atom of the same or different elements, are the smallest units of compounds. While these terms are related to matter, they represent structures that are more complex than a single atom. Hence, the atom stands out as the most fundamental unit of matter, making it the correct choice for this question.