Chemical compound

A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by mass
Fine-tuning the definition

There are some exceptions to the definition above. Certain crystalline compounds are non-stoichiometric compounds because they vary 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 compared to mixtures

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.

An example of a mixture that is often confused to be a compound is an alloy. It is made mechanically, most commonly by heating up all of the constituent(s) and then cooling it quickly so that the constituents are then "caught" in the base metal.

Chemists describe compounds using formula in various formats. For molecules, the formula for the molecular unit is shown. For polymeric materials, such as minerals and many metal oxides, the empirical formula is given, e.g. NaCl for table salt. The order of the elements in molecular and empirical formulas is C, then H and then alphabetical. Trifluoroacetic acid is thus described as C2HF3O2. More descriptive formulas convey structure information, illustrated again with trifluoroacetic acid. CF3CO2H. On the other hand, formulas for inorganic compounds often do not convey structural information, as illustrated by H2SO4 for a molecule that has no H-S bonds. A more descriptive presentation would be O2S(OH)2.

Formula
Elements form compounds to become more stable. They become stable when they have the maximum number of possible electrons in their outermost energy level, which is normally two or eight valence electrons. This is the reason that noble gases do not frequently react: they already possess eight valence electrons (the exception being helium, which requires only two valence electrons to achieve stability).