What element is this? how do you know?

Atom is the most basic unit of matter. They are the general term 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. For example, you have water; it has a chemical formula of H2O. If you separate H from O, you have two atoms. Two atoms for hydrogen and one atom for oxygen. However, they are of different elements. And when you combine them to form H2O, you create a molecule.