Here's why:
* Atom: An atom is the basic unit of an element. It's the smallest unit that can exist and still be considered that element. However, individual atoms often don't exist independently, especially in the context of forming a substance.
* Molecule: A molecule is formed when two or more atoms bond together. The resulting molecule has its own unique properties that may be very different from the individual atoms that make it up. For example, two hydrogen atoms bond to form a water molecule (H₂O), which is a liquid at room temperature, unlike the hydrogen atoms themselves which are gases.
Therefore, the smallest unit of a substance that retains all the chemical properties of the substance is the molecule.
