* Matter is defined as anything that has mass and takes up space. Atoms, the fundamental building blocks of all substances, possess both mass and occupy space.

* Atoms are composed of subatomic particles: protons, neutrons, and electrons. These particles contribute to the mass of the atom and determine its properties.

* Atoms combine to form molecules and larger structures: These molecules and structures also occupy space and have mass, further confirming that atoms are indeed matter.

Therefore, atoms are the smallest units of matter, and they are the foundation of all the matter we see and interact with in the universe.