Chemical compound

A chemical compound is a substance consisting of two or more different elements bonded together in a fixed mass ratio that can be split into simpler substances.^[1][2]

Contents 1 Nature 1.1 Fine-tuning the definition 1.1.1 Compounds compared to mixtures 2 Formula 3 Phases and thermal properties 4 CAS number 5 References

[] Nature

[] 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.

[] 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.

Nevertheless, there are multiple border cases between compound and mixture. Examples are solutions of alkali metals in liquid ammonia, intermetallic compounds, etc..

[] 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 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).

[] 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

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v - d - e Fields within the natural sciences Astronomy · Biology · Chemistry · Earth science · Physics

A chemical compound is a substance consisting of two or more different elements bonded together in a fixed mass ratio that can be split into simpler substances.^[1][2]

Contents 1 Nature 1.1 Fine-tuning the definition 1.1.1 Compounds compared to mixtures 2 Formula 3 Phases and thermal properties 4 CAS number 5 References

[] Nature

[] 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.

[] 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.

Nevertheless, there are multiple border cases between compound and mixture. Examples are solutions of alkali metals in liquid ammonia, intermetallic compounds, etc..

[] 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 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).

[] 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

v - d - e Chemistry Analytical chemistry - Biochemistry - Bioinorganic chemistry - Bioorganic chemistry - Chemical biology - Chemistry education - Click chemistry - Cluster chemistry - Computational chemistry - Electrochemistry - Environmental chemistry - Green chemistry - Inorganic chemistry - Materials science - Medicinal chemistry - Nuclear chemistry - Organic chemistry - Organometallic chemistry - Pharmacy - Physical chemistry - Photochemistry - Polymer chemistry - Solid-state chemistry - Supramolecular chemistry - Theoretical chemistry - Thermochemistry - Wet chemistry List of biomolecules - List of inorganic compounds - List of organic compounds - Periodic table

v - d - e Fields within the natural sciences Astronomy · Biology · Chemistry · Earth science · Physics