A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by mass
The atoms in the molecule can be held together by no bonds, covalent bonds or ionic bonds. For example H2O is held together by polar covalent bonds. Sodium chloride is an example of an ionic compound.
Fine-tuning the definition There are some exceptions to the definition above. Certain crystalline compounds may be treated as chemical compounds despite varying in composition according to the presence or otherwise of elements trapped within the crystal structure. Some compounds regarded as chemically identical may have varying amounts of heavy or light isotopes of the constituent elements, which will make the ratio of elements by mass vary slightly. A compound therefore may not be completely homogenous, but for most purposes in chemistry it can be regarded as such. Not all molecules are compounds. A diatomic molecule of hydrogen, represented by H2, is homonuclear — made of atoms of only one element, so is not regarded as a compound. Compounds are pure substances that contain two or more elements combined in a definite fixed proportion.
Compounds have different physical and chemical properties from their constituent elements. This is the one principal criterion for distinguishing a compound from a mixture of elements or substances: a mixture's properties are generally similar or related to the properties of its constituents. Another criterion is that the constituents of a mixture can usually be separated by simple, mechanical means such as filtering, those of a compound are often very hard to separate. Furthermore, when a compound is formed from its constituents, a chemical change takes place through chemical reactions. Mixtures can be made by mechanical means alone.