Here's why:
* Atom: An atom is the fundamental building block of matter, but it's not necessarily a complete substance. For example, a single oxygen atom doesn't have the properties of oxygen gas (O2).
* Molecule: A molecule is formed when two or more atoms bond together. It's the smallest unit that retains the chemical properties of the substance. For example, a water molecule (H2O) has the properties of water – it's liquid at room temperature, transparent, and can dissolve many substances.
Important Note: There are exceptions to this rule, especially with elements that exist as single atoms (noble gases like Helium, Neon, etc.). In these cases, the atom itself is the smallest unit that behaves like the substance.
