The small particle of substance that retains the characteristics of that substance is called a molecule.

Here's why:

* Molecules are the smallest unit of a substance that retains the chemical properties of that substance. They are made up of two or more atoms bonded together.

* Atoms are the building blocks of molecules, but they don't have the same properties as the substance they form. For example, a single oxygen atom is very different from an oxygen molecule (O2), which is what we breathe.

Let me know if you have any other questions!