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A chemical compound is a substance consisting of two or more elements chemically-bonded together in a fixed proportion by mass.^[1]

Nature of a chemical compound

Fixed composition

By the definition above, a compound is a chemical substance with a definite chemical composition. This arises because a compound is made up of identical molecules comprising atoms of the constituent elements, which is expressed by a chemical formula. For example, water is a compound consisting of molecules each made from two hydrogen atoms bonded to an oxygen atom, represented by the formula H₂O.

Bonds

Main article: Chemical bond

The atoms in the molecule can be held together by a variety of bonds, such as covalent bonds and 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 compared to mixtures

Compounds have different physical and chemical properties from their constituent elements. This is one the principal criteria 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 a when a compound is formed from its constituents, a chemical change takes place through chemical reactions. Mixtures can be made by mechanical means alone.

Formula

Main article: Chemical formula

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.

Elements form compounds to become more stable. They become stable when they have eight electrons in their outermost energy level (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).

Phases and thermal properties

Compounds may have several possible phases. All compounds can exist as solids, at least at low enough temperatures. Molecular compounds may also exist as liquids, gases, and, in some cases, even plasmas. All compounds decompose upon applying heat. The temperature at which such fragmentation occurs is often called the decomposition temperature. Decomposition temperatures are not sharp and depend on the rate of heating. At sufficiently high temperatures, all compounds, either after they have decomposed somehow or in the act of decomposing, fragment into smaller compounds or to individual atoms.

CAS number

Every chemical compound that has been described in the literature carries a unique numerical identifier, its CAS number.

See also

• Chemical bond

References

•  • [ e] Chemistry
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List of biomolecules • List of inorganic compounds • List of organic compounds • Periodic table

•  • [ e] Fields within the natural sciences
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