The smallest unit into which matter can be divided without the release of electrically charged particles. The atom is the basic building block of chemistry.

An element is a pure substance that consists of one type of atom.

A molecule is two or more atoms bonded together, this includes atoms that are the same (such as two oxygen atoms) and atoms that are different (such as one sodium atom (Na) and one chlorine atom (Cl)).

All compounds are molecules, but not all molecules are compounds.

A compound is a material formed by chemically bonding two or more different chemical elements. The type of bond keeping elements in a compound together may vary: covalent bonds and ionic bonds are two common types. The elements are always present in fixed ratios in any compound.