Atom is the most basic unit of matter. Atoms are used to describe pieces of matter, but a different kind of atom makes up an element. A combination of atom of different elements creates a molecule. Let's say you have some water, the chemical formula is "H20." If you wanted to separate H and O, that gives you two atoms. H would be Hydrogen and the other atom is Oxygen. So when we combine these two atom they formed as the chemical formula H20.
