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A 'chemical compound' is a chemical substance of two or more different chemically bonded chemical elements [citation needed], with a fixed ratio determining the composition. The ratio of each element is usually expressed by chemical formula. For example, water (H₂O) is a compound consisting of two hydrogen atoms bonded to an oxygen atom.
The atoms within a compound can be held together by a variety of interactions, ranging from covalent bonds to electrostatic forces in ionic bonds. A continuum of bond polarities exist between the purely covalent bond (as in H₂) and ionic bonds. For example H₂O is held together by polar covalent bonds. Sodium chloride is an example of an ionic compound.

Contents

Formula

Phases and thermal properties

CAS number

References

Formula

Main articles: 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.

References