Here's why:

* Pure Substance: A pure substance is made up of only one type of particle. Examples include elements like gold (Au) or compounds like water (H₂O).

* Atom: An atom is the basic unit of an element. It's the smallest particle that still retains the chemical properties of that element. Atoms are made up of protons, neutrons, and electrons.

* Molecules: While atoms are the fundamental building blocks, some pure substances exist as molecules. A molecule is formed when two or more atoms bond together. For example, a water molecule is made of two hydrogen atoms bonded to one oxygen atom (H₂O).

So, while molecules might be the smallest units of a compound, atoms are the ultimate building blocks of all matter, including pure substances.