chemical compound is a chemical substance composed of many identical molecules (or molecular entities) composed of atoms from more than one element held together by chemical bonds. A chemical element bonded to an identical chemical element is not a chemical compound since only one element, not two different elements, is involved.

There are four types of compounds, depending on how the constituent atoms are held together:

molecules held together by covalent bonds
ionic compounds held together by ionic bonds
intermetallic compounds held together by metallic bonds
certain complexes held together by coordinate covalent bonds.