Here's the key difference:

* Atoms are the fundamental building blocks of matter. They are the smallest unit of an element that retains the chemical properties of that element. Examples: Carbon (C), Oxygen (O), Hydrogen (H).

* Molecules are formed when two or more atoms bond together. These atoms can be of the same element (like O2, oxygen gas) or different elements (like H2O, water).

Think of it this way:

* An atom is like a Lego brick.

* A molecule is like a structure built from multiple Lego bricks.

So, a molecule is made up of atoms, but an atom is not a molecule.