In chemistry, a compound is a pure chemical substance made up of two or more different elements. For example, molecular hydrogen with chemical formula H2 is not considered a compound as it is composed of only one type of element: hydrogen. However, water with chemical formula H2O is considered a compound as it is made of two parts hydrogen and one part oxygen. Thus, all compounds are molecules, but not all molecules are compounds.
In terms of bonding, compounds are formed when two or more atoms are joined chemically, meaning a defined chemical bond is present between linked atoms. This differentiates a compound from a mixture, which is just a substance made by combining two or more different elements in such a way that no chemical reaction occurs. Thus, compounds can usually be separated into its constituting atoms by chemical reactions, while mixtures do not have this ability.
Structurally, since compounds have a fixed ratio of atoms, their spatial arrangement of bonds and atoms are well-defined. They can vary not only on the type of composing atoms, but also on the type of bonds holding the compound together. As with molecules, compounds can be held together by covalent bonds, ionic bonds, metallic bonds, and coordinate covalent bonds. Thus, understanding the nature of compounds is essential for constructing a conceptual framework needed to understand chemical composition.