What differentiates atoms from elements?

Oxygen (for example) is an element (pure atom) while an atom can be more than one element (an atom of H2O for example is more than one element), and there can be atoms of an element (like an atom of Oxygen).

A particular atom will have the same number of protons and electrons and most atoms have at least as many neutrons as protons. An element is a substance that is made entirely from one type of atom. For example, the element hydrogen is made from atoms containing just one proton and one electron.